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Hamiltonian matrices in electronic and nuclear contexts are highly computation intensive to calculate, mainly due to the cost for the potential matrix. Typically, these matrices contain many off-diagonal elements that are orders of magnitude smaller than diagonal elements. We illustrate that here for vibrational H-matrices of H2O, C2H3 (vinyl), and C2H5NO2 (glycine) using full-dimensional ab initio-based potential surfaces. We then show that many of these small elements can be replaced by zero with small errors of the resulting full set of eigenvalues, depending on the threshold value for this replacement. As a result of this empirical evidence, we investigate three machine learning approaches to predict the zero elements. This is shown to be successful for these H-matrices after training on a small set of calculated elements. For H-matrices of vinyl and glycine, of order 15 552 and 8828, respectively, training on a percent or so of elements is sufficient to obtain all eigenvalues with a mean absolute error of roughly 2 cm−1. [ABSTRACT FROM AUTHOR]

We wish to describe a potential energy surface by using a basis of permutationally invariant polynomials whose coefficients will be determined by numerical regression so as to smoothly fit a dataset of electronic energies as well as, perhaps, gradients. The polynomials will be powers of transformed internuclear distances, usually either Morse variables, exp(−ri,j/λ), where λ is a constant range hyperparameter, or reciprocals of the distances, 1/ri,j. The question we address is how to create the most efficient basis, including (a) which polynomials to keep or discard, (b) how many polynomials will be needed, (c) how to make sure the polynomials correctly reproduce the zero interaction at a large distance, (d) how to ensure special symmetries, and (e) how to calculate gradients efficiently. This article discusses how these questions can be answered by using a set of programs to choose and manipulate the polynomials as well as to write efficient Fortran programs for the calculation of energies and gradients. A user-friendly interface for access to monomial symmetrization approach results is also described. The software for these programs is now publicly available. [ABSTRACT FROM AUTHOR]

There has been great progress in developing methods for machine-learned potential energy surfaces. There have also been important assessments of these methods by comparing so-called learning curves on datasets of electronic energies and forces, notably the MD17 database. The dataset for each molecule in this database generally consists of tens of thousands of energies and forces obtained from DFT direct dynamics at 500 K. We contrast the datasets from this database for three "small" molecules, ethanol, malonaldehyde, and glycine, with datasets we have generated with specific targets for the potential energy surfaces (PESs) in mind: a rigorous calculation of the zero-point energy and wavefunction, the tunneling splitting in malonaldehyde, and, in the case of glycine, a description of all eight low-lying conformers. We found that the MD17 datasets are too limited for these targets. We also examine recent datasets for several PESs that describe small-molecule but complex chemical reactions. Finally, we introduce a new database, "QM-22," which contains datasets of molecules ranging from 4 to 15 atoms that extend to high energies and a large span of configurations. [ABSTRACT FROM AUTHOR]

Permutationally invariant polynomial (PIP) regression has been used to obtain machine-learned potential energy surfaces, including analytical gradients, for many molecules and chemical reactions. Recently, the approach has been extended to moderate size molecules with up to 15 atoms. The algorithm, including "purification of the basis," is computationally efficient for energies; however, we found that the recent extension to obtain analytical gradients, despite being a remarkable advance over previous methods, could be further improved. Here, we report developments to further compact a purified basis and, more significantly, to use the reverse differentiation approach to greatly speed up gradient evaluation. We demonstrate this for our recent four-body water interaction potential. Comparisons of training and testing precision on the MD17 database of energies and gradients (forces) for ethanol against numerous machine-learning methods, which were recently assessed by Dral and co-workers, are given. The PIP fits are as precise as those using these methods, but the PIP computation time for energy and force evaluation is shown to be 10–1000 times faster. Finally, a new PIP potential energy surface (PES) is reported for ethanol based on a more extensive dataset of energies and gradients than in the MD17 database. Diffusion Monte Carlo calculations that fail on MD17-based PESs are successful using the new PES. [ABSTRACT FROM AUTHOR]

"Δ-machine learning" refers to a machine learning approach to bring a property such as a potential energy surface (PES) based on low-level (LL) density functional theory (DFT) energies and gradients close to a coupled cluster (CC) level of accuracy. Here, we present such an approach that uses the permutationally invariant polynomial (PIP) method to fit high-dimensional PESs. The approach is represented by a simple equation, in obvious notation VLL→CC = VLL + ΔVCC–LL, and demonstrated for CH4, H3O+, and trans and cis-N-methyl acetamide (NMA), CH3CONHCH3. For these molecules, the LL PES, VLL, is a PIP fit to DFT/B3LYP/6-31+G(d) energies and gradients and ΔVCC–LL is a precise PIP fit obtained using a low-order PIP basis set and based on a relatively small number of CCSD(T) energies. For CH4, these are new calculations adopting an aug-cc-pVDZ basis, for H3O+, previous CCSD(T)-F12/aug-cc-pVQZ energies are used, while for NMA, new CCSD(T)-F12/aug-cc-pVDZ calculations are performed. With as few as 200 CCSD(T) energies, the new PESs are in excellent agreement with benchmark CCSD(T) results for the small molecules, and for 12-atom NMA, training is done with 4696 CCSD(T) energies. [ABSTRACT FROM AUTHOR]

A full-dimensional, permutationally invariant potential energy surface (PES) for the glycine amino acid is reported. A precise fit to energies and gradients calculated at the density functional theory (DFT)/B3LYP level of electronic-structure theory with Dunning's aug-cc-pVDZ basis set is performed involving 20 000 low-energy points and associated Cartesian gradients plus about 50 000 additional higher-energy points. The fact that newly calculated DFT/B3LYP energies for the main stationary points are close to the coupled-cluster single-double-triple [CCSD(T)] values, recently reported in the literature, provides reassurance about the accuracy of the constructed PES. Eight conformers and numerous saddle points are identified and characterized by describing geometries, relative stability, and harmonic frequencies. Stochastic and dynamical approaches are employed to study the vibrational ground state. Specifically, diffusion Monte Carlo simulations and approximate quantum dynamics, performed by means of the adiabatic switching semiclassical initial value representation technique, provide zero-point energies in excellent agreement with each other. The PES we report is sufficiently complete to permit spectroscopic and dynamical studies on glycine, which may be of interest to the biochemical and astrochemistry communities. [ABSTRACT FROM AUTHOR]

The aqueous proton is a common and long-studied species in chemistry, yet there is currently intense interest devoted to understanding its hydration structure and transport dynamics. Typically described in terms of two limiting structures observed in gas-phase clusters, the Zundel H5O2+ and Eigen H9O4+ ions, the aqueous structure is less clear due to the heterogeneity of hydrogen bonding environments and room-temperature structural fluctuations in water. The linear infrared (IR) spectrum, which reports on structural configurations, is challenging to interpret because it appears as a continuum of absorption, and the underlying vibrational modes are strongly anharmonically coupled to each other. Recent two-dimensional IR (2D IR) experiments presented strong evidence for asymmetric Zundel-like motifs in solution, but true structure–spectrum correlations are missing and complicated by the anharmonicity of the system. In this study, we employ high-level vibrational self-consistent field/virtual state configuration interaction calculations to demonstrate that the 2D IR spectrum reports on a broad distribution of geometric configurations of the aqueous proton. We find that the diagonal 2D IR spectrum around 1200 cm−1 is dominated by the proton stretch vibrations of Zundel-like and intermediate geometries, broadened by the heterogeneity of aqueous configurations. There is a wide distribution of multidimensional potential shapes for the proton stretching vibration with varying degrees of potential asymmetry and confinement. Finally, we find specific cross peak patterns due to aqueous Zundel-like species. These studies provide clarity on highly debated spectral assignments and stringent spectroscopic benchmarks for future simulations. [ABSTRACT FROM AUTHOR]

Energy transfer between vibrational modes can be quite facile, and it has been proposed as the dominant mechanism for energy pooling in extreme environments such as nonthermal plasmas and laser cavities. To understand such processes, we perform quasi-classical trajectory studies of CO(v) + CO(v) collisions on a new full-dimensional potential energy surface fit to high-level ab initio data using a neural network method and examine the key vibrational energy transfer channels. In addition to the highly efficient CO(v + 1) + CO(v − 1) channel, there exists a significant, sometimes dominant, CO(v + 2) + CO(v − 2) channel for large v states at low collision energies. The latter is shown to stem from the substantially increased interaction between highly vibrationally excited CO, which has a much larger dipole moment than at its equilibrium bond length. Finally, the vibrational state-specific cross sections and their energy dependence on the thermal range are predicted from a limited dataset using Gaussian process regression. The relevance of these results to plasma chemistry and laser engineering and the recently observed flipping of highly vibrationally excited CO adsorbates on a cold NaCl surface is discussed. [ABSTRACT FROM AUTHOR]

We report permutationally invariant polynomial (PIP) fits to energies and gradients for 15-atom tropolone. These include standard, augmented, and fragmented PIP bases. Approximately, 6600 energies and their associated gradients are obtained from direct-dynamics calculations using DFT/B3LYP/6-31+G(d) supplemented by grid calculations spanning an energy range up to roughly 35 000 cm−1. Three fragmentation schemes are investigated with respect to efficiency and fit precision. In addition, several fits are done with reduced weight for gradient data relative to energies. These do result in more precision for the H-transfer barrier height. The properties of the fits such as stationary points, harmonic frequencies, and the barrier to H-atom transfer are reported and compared to direct calculations. A previous 1D model is used to obtain the tunneling splitting for the ground vibrational state and qualitative predictions for excited vibrational states. This model is applied to numerous fits with different barrier heights and then used to extrapolate the H and D atom tunneling splittings to values at the CCSD(T)-F12 barrier. The extrapolated values are 2.3 and 0.14 cm−1, respectively for H and D. These are about a factor of two larger than experiment, but within the expected level of agreement with experiment for the 1D method used and the level of the electronic structure theory. [ABSTRACT FROM AUTHOR]

We have employed extended multiconfiguration quasidegenerate perturbation theory, fourfold-way diabatic molecular orbitals, and configurational uniformity to develop a global three-state diabatic representation of the potential energy surfaces and their couplings for the electronically nonadiabatic reaction OH* + H2 → H2O + H, where * denotes electronic excitation to the A 2Σ+ state. To achieve sign consistency of the computed diabatic couplings, we developed a graphics processing unit-accelerated algorithm called the cluster-growing algorithm. Having obtained consistent signs of the diabatic couplings, we fit the diabatic matrix elements (which consist of the diabatic potentials and the diabatic couplings) to analytic representations. Adiabatic potential energy surfaces are generated by diagonalizing the 3 × 3 diabatic potential energy matrix. The comparisons between the fitted and computed diabatic matrix elements and between the originally computed adiabatic potential energy surfaces and those generated from the fits indicate that the current fit is accurate enough for dynamical studies, and it may be used for quantal or semiclassical dynamics calculations. [ABSTRACT FROM AUTHOR]

We report full and fragmented potential energy surfaces (PESs) for N-methyl acetamide that contain the cis and trans isomers and the saddle points separating them. The full PES uses Permutationally Invariant Polynomials (PIPs) in reduced symmetry which describe the three-fold symmetry of each methyl rotor. A more efficient PES is an extension of the fragmented PIP approach we reported recently. In this approach, the set of Morse variables is partitioned and the fragmented PIP basis is the union of the PIP basis for each set of variables. This approach is general and can be used with neural network fits. The fits are done using roughly 250 000 electronic energies and gradients obtained from direct dynamics, using the B3LYP/cc-pVDZ level of theory. The full PIP basis in 66 Morse variables, with a maximum polynomial order of 3, contains 8040 linear coefficients. The fragmented PIP basis, also with a maximum polynomial order of 3, contains 6121 coefficients. The root-mean-square errors of both PESs are roughly 100 cm−1 for energies and 15 cm−1/bohr per atom for gradients, for energies up to roughly 45 000 cm−1, relative to the trans minimum. Energies and normal mode frequencies of the cis and trans isomers for the full and fragmented PESs agree well with direct calculations. The energies of the two saddle points separating these minima are precisely given by both PESs. Diffusion Monte Carlo calculations of the zero-point energies of the two isomers are also reported. [ABSTRACT FROM AUTHOR]

We describe and apply a method to extend permutationally invariant polynomial (PIP) potential energy surface (PES) fitting to molecules with more than 10 atoms. The method creates a compact basis of PIPs as the union of PIPs obtained from fragments of the molecule. An application is reported for trans-N-methyl acetamide, where B3LYP/cc-pVDZ electronic energies and gradients are used to develop a full-dimensional potential for this prototype peptide molecule. The performance of several fragmented bases is verified against a benchmark PES using all (66) Morse variables. The method appears feasible for much larger molecules. [ABSTRACT FROM AUTHOR]

We present high-level, coupled-mode calculations of the infrared spectrum of the cyclic formic acid dimer. The calculations make use of full-dimensional, ab initio potential energy and dipole moment surfaces. The potential is a linear least-squares fit to 13 475 CCSD(T)-F12a/haTZ (haTZ means aug-cc-pVTZ basis set for O and C, and cc-pVTZ for H) energies, and the dipole moment surface is a fit to the dipole components, calculated at the MP2/haTZ level of theory. The variables of both fits are all (45) internuclear distances (actually Morse variables). The potential, which is fully permutationally invariant, is the one published recently and the dipole moment surface is newly reported here. Details of the fits, especially the dipole moment, and the database of configurations are given. The infrared spectrum of the dimer is calculated by solving the nuclear Schrödinger equation using a vibrational self-consistent field and virtual-state configuration interaction method, with subsets of the 24 normal modes, up to 15 modes. The calculations indicate strong mode-coupling in the C—H and O—H stretching region of the spectrum. Comparisons are made with experiments and the complexity of the experimental spectrum in the C—H and O—H stretching region is successfully reproduced. [ABSTRACT FROM AUTHOR]

The cyclic HCl–(H2O)3 tetramer is the largest observed neutral HCl–(H2O)n cluster. The vibrational predissociation of HCl–(H2O)3 is investigated by theory, quasiclassical trajectory (QCT) calculations, and experiment, following the infrared excitation of the hydrogen-bonded OH-stretch fundamental. The energetically possible dissociation pathways are HCl + (H2O)3 (Pathway 1) and H2O + HCl–(H2O)2 (Pathway 2). The HCl and H2O monomer fragments are observed by 2 + 1 resonance enhanced multiphoton ionization combined with time-of-flight mass spectrometry, and their rotational energy distributions are inferred and compared to the theoretical results. Velocity map images of the monomer fragments in selected rotational levels are used for each pathway to obtain pair-correlated speed distributions. The fragment speed distributions obtained by experiment and QCT calculations are broad and structureless, encompassing the entire range of allowed speeds for each pathway. Bond dissociation energies, D0, are estimated experimentally from the endpoints of the speed distributions: 2100 ± 300 cm−1 and 2400 ± 100 cm−1 for Pathway 1 and Pathway 2, respectively. These values are lower but in the same order as the corresponding calculated D0: 2426 ± 23 cm−1 and 2826 ± 19 cm−1. The differences are attributed to contributions from vibrational hot bands of the clusters that appear in the high-speed tails of the experimental pair-correlated distributions. Satisfactory agreement between theory and experiment is achieved when comparing the monomer fragments’ rotational energies, the shapes of the speed distributions, and the average fragment speeds and center-of-mass translational energies. Insights into the dissociation mechanism and lifetime are gained from QCT calculations performed on a previously reported many-body potential energy surface. It is concluded that the dissociation lifetime is on the order of 10 ps and that the final trimer products are in their lowest energy cyclic forms. [ABSTRACT FROM AUTHOR]

We report an ab initio, full-dimensional, potential energy surface (PES) for CO2--H2O, in which twobody interaction energies are fit using a basis of permutationally invariant polynomials and combined with accurate potentials for the non-interacting monomers. This approach which we have termed "plug and play" is extended here to improve the precision of the 2-body fit in the long range. This is done by combining two separate fits. One is a fit to 47 593 2-body energies in the region of strong interaction and approaching the long range, and the second one is a fit to 6244 2-body energies in the long range. The two fits have a region of overlap which permits a smooth switch from one to the other. All energies are obtained at the CCSD(T)-F12b/aug-cc-pVTZ level of theory. Properties of the full PES, i.e., stationary points, harmonic frequencies of the global minimum, etc., are shown to be in excellent agreement with direct CCSD(T)-F12b/aug-cc-pVTZ results. Diffusion Monte Carlo calculations of the dimer zeropoint energy (ZPE) are performed, and a dissociation energy, D0, of 787 cm-1 is obtained using that ZPE, De, and the rigorous ZPEs of the monomers. Using a benchmark De, D0 is 758 cm-1. Vibrational self-consistent field (VSCF)/virtual state configuration interaction (VCI) MULTIMODE calculations of intramolecular fundamentals are reported and are in good agreement with available experimental results. Finally, the full dimer PES is combined with an existing ab initio water potential to develop a potential for the CO2 hydrate clathrate CO2(H2O)20(512 water cage). A full normal-mode analysis of this hydrate clathrate is reported as are local-monomer VSCF/VCI calculations of the fundamentals of CO2. [ABSTRACT FROM AUTHOR]

The unimolecular dissociation dynamics of the dimethyl-substituted Criegee intermediate (CH3)2COO is examined experimentally using velocity map imaging to ascertain the translational and internal energy distributions of the OH and H2CC(CH3)O radical products. The energy profile of key features along the reaction coordinate is also evaluated theoretically. Unimolecular decay of (CH3)2COO is initiated by vibrational activation in the CH stretch overtone region and the resultant OH X²Π3/2 (v = 0) products are state-selectively ionized and imaged. Analysis reveals an isotropic spatial distribution, indicative of a 3 ps lower limit for the timescale of dissociation, and a broad and unstructured total kinetic energy release distribution. The energy released to products is partitioned principally as internal excitation of the H2CC(CH3)O fragments with modest translational excitation of the fragments and a small degree of OH rotational excitation. The total kinetic energy release distribution observed for (CH3)2COO is compared with that predicted for statistical partitioning over product quantum states, and contrasted with recent experimental and quasi-classical trajectory results for syn-CH3CHOO [N. M. Kidwell et al., Nat. Chem. 8, 509 (2016)]. [ABSTRACT FROM AUTHOR]

We report full-dimensional, ab initio-based potentials and dipole moment surfaces for NaCl, NaF, Na+H2O, F-H2O, and Cl-H2O. The NaCl and NaF potentials are diabatic ones that dissociate to ions. These are obtained using spline fits to CCSD(T)/aug-cc-pV5Z energies. In addition, non-linear least square fits using the Born-Mayer-Huggins potential are presented, providing accurate parameters based strictly on the current ab initio energies. The long-range behavior of the NaCl and NaF potentials is shown to go, as expected, accurately to the point-charge Coulomb interaction. The three ion-H2O potentials are permutationally invariant fits to roughly 20 000 coupled cluster CCSD(T) energies (awCVTZ basis for Na+ and aVTZ basis for Cl- and F-), over a large range of distances and H2O intramolecular configurations. These potentials are switched accurately in the long range to the analytical ion-dipole interactions, to improve computational efficiency. Dipole moment surfaces are fits to MP2 data; for the ion-ion cases, these are well described in the intermediate- and long-range by the simple point-charge expression. The performance of these new fits is examined by direct comparison to additional ab initio energies and dipole moments along various cuts. Equilibrium structures, harmonic frequencies, and electronic dissociation energies are also reported and compared to direct ab initio results. These indicate the high fidelity of the new PESs. [ABSTRACT FROM AUTHOR]

New, full-dimensional potential energy surfaces (PESs), obtained using precise least-squares fitting of high-level electronic energy databases, are reported for intrinsic H2(H2O) two-body and H2(H2O)2 three-body potentials. The database for H2(H2O) consists of approximately 44 000 energies at the coupled cluster singles and doubles plus perturbative triples (CCSD(T))-F12a/haQZ (aug-cc-pVQZ for O and cc-pVQZ for H) level of theory, while the database for the three-body interaction consists of more than 36 000 energies at the CCSD(T)-F12a/haTZ (aug-cc-pVTZ for O, cc-pVTZ for H) level of theory. Two precise potentials are based on the invariant-polynomial technique and are compared to computationally faster ones obtained via "purified" symmetrization. All fits use reduced permutational symmetry appropriate for these non-covalent interactions. These intrinsic potentials are employed together with existing ones for H2, H2O, and (H2O)2, to obtain full PESs for H2(H2O) and H2(H2O)2. Properties of these full PESs are presented, including a diffusion Monte Carlo calculation of the zero-point energy and wavefunction, and dissociation energy of the H2(H2O) dimer. These PESs together with an existing one for water clusters are used in a many-body representation of the PES of hydrogen clathrate hydrates, illustrated for H2@(H2O)20. An analysis of this hydrate is presented, including the electronic dissociation energy to remove H2 from the calculated equilibrium structure. [ABSTRACT FROM AUTHOR]

Even though quartic force fields (QFFs) and highly accurate coupled cluster computations describe the OCHCO+ cation at equilibrium as a complex between carbon monoxide and the formyl cation, two notable and typical interstellar and atmospheric molecules, the prediction from the present study is that the equilibrium C8v structure is less relevant to observables than the saddle-point D∞h structure. This is the conclusion from diffusion Monte Carlo and vibrational self-consistent field/virtual state configuration interaction calculations utilizing a semi-global potential energy surface. These calculations demonstrate that the proton "rattle" motion (v6) has centrosymmetric delocalization of the proton over the D∞h barrier lying only 393.6 cm-1 above the double-well OCHCO+ C∞v minima. As a result, this molecule will likely appear D8h, and the rotational spectrum will be significantly dimmer than the computed equilibrium 2.975 D center-of-mass dipole moment indicates. However, the proton transfer fundamental, determined to be at roughly 300 cm-1, has a very strong intensity. This prediction as well as those of other fundamentals should provide useful guides for laboratory detection of this cation. Finally, it is shown that the two highest energy QFF-determined modes are actually in good agreement with their vibrational configuration interaction counterparts. These high-level quantum chemical methods provide novel insights into this fascinating and potentially common interstellar molecule. [ABSTRACT FROM AUTHOR]

The calculation and characterization of the IR spectrum of liquid water have remained a challenge for theory. In this paper, we address this challenge using a combination of ab initio approaches, namely, a quantum treatment of IR spectrum using the ab initio WHBB water potential energy surface and a refined ab initio dipole moment surface. The quantum treatment is based on the embedded local monomer method, in which the three intramolecular modes of each embedded H2O monomer are fully coupled and also coupled singly to each of six intermolecular modes. The new dipole moment surface consists of a previous spectroscopically accurate 1-body dipole moment surface and a newly fitted ab initio intrinsic 2-body dipole moment. A detailed analysis of the new dipole moment surface in terms of the coordinate dependence of the effective atomic charges is done along with tests of it for the water dimer and prism hexamer double-harmonic spectra against direct ab initio calculations. The liquid configurations are taken from previous molecular dynamics calculations of Skinner and co-workers, using the TIP4P plus E3B rigid monomer water potential. The IR spectrum of water at 300 K in the range of 0-4000 cm-1 is calculated and compared with experiment, using the ab initio WHBB potential and new ab initio dipole moment, the q-TIP4P/F potential, which has a fixed-charged description of the dipole moment, and the TTM3-F potential and dipole moment surfaces. The newly calculated ab initio spectrum is in very good agreement with experiment throughout the above spectral range, both in band positions and intensities. This contrasts to results with the other potentials and dipole moments, especially the fixed-charge q-TIP4P/F model, which gives unrealistic intensities. The calculated ab initio spectrum is analyzed by examining the contribution of various transitions to each band. [ABSTRACT FROM AUTHOR]

A full-dimensional, global ab initio potential energy surface (PES) for the Ar-HOCO system is presented. The PES consists of a previous intramolecular ab initio PES for HOCO [J. Li, C. Xie, J. Ma, Y. Wang, R. Dawes, D. Xie, J. M. Bowman, and H. Guo, J. Phys. Chem. A 116, 5057 (2012)], plus a new permutationally invariant interaction potential based on fitting 12 432 UCCSD(T)-F12a/aVDZ counterpoise-corrected energies. The latter has a total rms fitting error of about 25 cm-1 for fitted interaction energies up to roughly 12 000 cm-1. Two additional fits are presented. One is a novel very compact permutational invariant representation, which contains terms only involving the Ar-atom distances. The rms fitting error for this fit is 193 cm-1. The other fit is the widely used pairwise one. The pairwise fit to the entire data set has an rms fitting error of 427 cm-1. All of these potentials are used in preliminary classical trajectory calculations of energy transfer with a focus on comparisons with the results using the benchmark potential. [ABSTRACT FROM AUTHOR]

We combine the techniques of infrared multiphoton dissociation (IRMPD) with state selective ion imaging to probe roaming dynamics in the unimolecular dissociation of nitromethane and methyl nitrite. Recent theoretical calculations suggest a "roaming-mediated isomerization" pathway of nitromethane to methyl nitrite prior to decomposition. State-resolved imaging of the NO product coupled with infrared multiphoton dissociation was carried out to examine this unimolecular decomposition near threshold. The IRMPD images for the NO product from nitromethane are consistent with the earlier IRMPD studies that first suggested the importance of an isomerization pathway. A significant Λ-doublet propensity is seen in nitromethane IRMPD but not methyl nitrite. The experimental observations are augmented by quasiclassical trajectory calculations for nitromethane and methyl nitrite near threshold for each dissociation pathway. The observation of distinct methoxy vibrational excitation for trajectories from nitromethane and methyl nitrite dissociation at the same total energy show that the nitromethane dissociation bears a nonstatistical signature of the roaming isomerization pathway, and this is possibly responsible for the nitromethane Λ-doublet propensity as well. [ABSTRACT FROM AUTHOR]

We report a CCSD(T)/cc-pCV5Z quartic force field (QFF) and a semi-global CCSD(T)-F12b/aug-cc-pVTZ potential energy surface (PES) for singlet, cyclic C4. Vibrational fundamentals, combinations, and overtones are obtained using vibrational second-order perturbation theory (VPT2) and the vibrational configuration-interaction (VCI) approach. Agreement is within 10 cm-1 between the VCI calculated fundamentals on the QFF and PES using the MULTIMODE (MM) program, and VPT2 and VCI results agree for the fundamentals. The agreement between VPT2-QFF and MM-QFF results is also good for the C4 combinations and overtones. The J = 1 and J = 2 rovibrational energies are reported from both VCI (MM) on the PES and VPT2 on the QFF calculations. The spectroscopic constants of 12C4 and two C2v-symmetry, single 13C-substituted isotopologues are presented, which may help identification of cyclic C4 in future experimental analyses or astronomical observations. [ABSTRACT FROM AUTHOR]

We present a theory of mode-specific tunneling that makes use of the general tunneling path along the imaginary-frequency normal mode of the saddle point, Qim, and the associated relaxed potential, V(Qim) [Y. Wang and J. M. Bowman, J. Chem. Phys. 129, 121103 (2008)]. The novel aspect of the theory is the projection of the normal modes of a minimum onto the Qim path and the determination of turning points on V(Qim). From that projection, the change in tunneling upon mode excitation can be calculated. If the projection is zero, no enhancement of tunneling is predicted. In that case vibrationally adiabatic (VA) theory could apply. However, if the projection is large then VA theory is not applicable. The approach is applied to mode-specific tunneling in full-dimensional malonaldehyde, using an accurate full-dimensional potential energy surface. Results are in semi-quantitative agreement with experiment for modes that show large enhancement of the tunneling, relative to the ground state tunneling splitting. For the six out-of-plane modes, which have zero projection on the planar Qim path, VA theory does apply, and results from that theory agree qualitatively and even semi-quantitatively with experiment. We also verify the failure of simple VA theory for modes that show large enhancement of tunneling. [ABSTRACT FROM AUTHOR]

Large-scale, rovibrational variational calculations are performed for ethylene, using the potential energy surface published by Avila and Carrington [J. Chem. Phys. 135, 064101 (2011)]. Energies for J = 0 are in very good agreement with their benchmark results. Corresponding energies for J = 1 and J = 2 are also given. Calculations with a slightly reduced basis permit energies to J = 40, allowing a reliable determination of the partition function at 296 K. Using a new ab initio dipole moment surface, reported here, the infrared spectra of five dipole-allowed fundamentals are calculated. Both the partition function and infrared spectra are shown to be in excellent agreement with those in the experimental HITRAN database, with the exception of one band, which we believe is partially mis-assigned in HITRAN. [ABSTRACT FROM AUTHOR]

Full-dimensional ab initio potential energy surface is constructed for the H7+ cluster. The surface is a fit to roughly 160 000 interaction energies obtained with second-order MöllerPlesset perturbation theory and the cc-pVQZ basis set, using the invariant polynomial method [B. J. Braams and J. M. Bowman, Int. Rev. Phys. Chem. 28, 577 (2009)]. We employ permutationally invariant basis functions in Morse-type variables for all the internuclear distances to incorporate permutational symmetry with respect to interchange of H atoms into the representation of the surface. We describe how different configurations are selected in order to create the database of the interaction energies for the linear least squares fitting procedure. The root-mean-square error of the fit is 170 cm-1 for the entire data set. The surface dissociates correctly to the H5+ + H2 fragments. A detailed analysis of its topology, as well as comparison with additional ab initio calculations, including harmonic frequencies, verify the quality and accuracy of the parameterized potential. This is the first attempt to present an analytical representation of the 15-dimensional surface of the H7+ cluster for carrying out dynamics studies. [ABSTRACT FROM AUTHOR]

Translationally hot H collisions with the acetylene are investigated using quasiclassical trajectory calculations, on a recent full-dimensional ab initio-based potential energy surface. Three outcomes are focused on: non-reactive energy transfer via prompt collisions, non-reactive energy transfer via the formation of the vinyl complex, and reactive chemical H-atom exchange, also via complex formation. The details of these outcomes are presented and correlated with the collision lifetime. Large energy transfer is found via complex formation, which can subsequently decay back to reactants, a non-reactive event, or to new products, a reactive event. For the present system, these two events are experimentally indistinguishable. [ABSTRACT FROM AUTHOR]

We present extensions to the local-monomer (LMon) Model, a general quantum method to describe coupled intramolecular vibrational modes of a molecular cluster consisting of a set of monomers [Y. Wang and J. M. Bowman, J. Chem. Phys. 134, 154510 (2011)], to incorporate monomer-monomer coupling. A central aspect of the LMon model is a local normal-mode analysis, done for each monomer, perturbed by all other mononers. Monomer-monomer coupling is described by several approaches based on these normal-mode analyses. Two are Hückel-type models, where coupling constants for each intramolecular mode are determined non-empirically from normal-mode analyses. One model, the simple one, is limited to nearest-neighbor interactions. The second and more general one determines monomer-monomer couplings from the full and local-monomer Hessians, with no further assumptions. The simple approach is applied to the water tetramer, pentamer and ring hexamer. For the tetramer and ring hexamer cases, artificial degeneracies of the intramolecular energies in the LMon model, owing to the high symmetry of the cluster, are correctly lifted. The general approach to obtain coupling constants is illustrated for the ring hexamer, where new fundamental energies are reported. Other, more rigorous approaches are suggested but not implemented. [ABSTRACT FROM AUTHOR]

We report a high-quality, ab initio, full-dimensional global potential energy surface (PES) for the Cl(2P, 2P3/2) + CH4 reaction, which describes both the abstraction (HCl + CH3) and substitution (H + CH3Cl) channels. The analytical PES is a least-squares fit, using a basis of permutationally invariant polynomials, to roughly 16 000 ab initio energy points, obtained by an efficient composite method, including counterpoise and spin-orbit corrections for the entrance channel. This composite method is shown to provide accuracy almost equal to all-electron CCSD(T)/aug-cc-pCVQZ results, but at much lower computational cost. Details of the PES, as well as additional high-level benchmark characterization of structures and energetics are reported. The PES has classical barrier heights of 2650 and 15 060 cm-1 (relative to Cl(2P3/2) + CH4(eq)), respectively, for the abstraction and substitution reactions, in good agreement with the corresponding new computed benchmark values, 2670 and 14 720 cm-1. The PES also accurately describes the potential wells in the entrance and exit channels for the abstraction reaction. Quasiclassical trajectory calculations using the PES show that (a) the inclusion of the spin-orbit corrections in the PES decreases the cross sections by a factor of 1.5-2.5 at low collision energies (Ecoll); (b) at Ecoll ≈ 13 000 cm-1 the substitution channel opens and the H/HCl ratio increases rapidly with Ecoll; (c) the maximum impact parameter (bmax) for the abstraction reaction is ∼6 bohr; whereas bmax is only ∼2 bohr for the substitution; (d) the HCl and CH3 products are mainly in the vibrational ground state even at very high Ecoll; and (e) the HCl rotational distributions are cold, in excellent agreement with experiment at Ecoll = 1280 cm-1. [ABSTRACT FROM AUTHOR]

We report a chemically accurate global potential energy surface for the HOCO system based on high-level ab initio calculations at ∼35 000 points. The potential energy surface is shown to reproduce important stationary points and minimum energy paths. Quasi-classical trajectory calculations indicated a good agreement with experimental data. [ABSTRACT FROM AUTHOR]

We report tunneling splittings associated with the large amplitude 1,2 H-atom migration to the global minima in the vinyl radical. These are obtained using a recent full-dimensional ab initio potential energy surface (PES) [A. R. Sharma, B. J. Braams, S. Carter, B. C. Shepler, and J. M. Bowman, J. Chem. Phys. 130(17), 174301 (2009)] and independently, directly calculated 'reaction paths.' The PES is a multidimensional fit to coupled cluster single and double and perturbative treatment of triple excitations coupled-cluster single double triple (CCSD(T)) with the augmented correlation consistent triple zeta basis set (aug-cc-pVTZ). The reaction path potentials are obtained from a series of CCSD(T)/aug-cc-pVnTZ calculations extrapolated to the complete basis set limit. Approximate 1D calculations of the tunneling splitting for these 1,2-H atom migrations are obtained using each of these potentials as well as quite different 1D Hamiltonians. The splittings are calculated over a large energy ranges, with results from the two sets of calculations in excellent agreement. Though negligibly slow (>1 s) for the vibrational ground state, this work predicts tunneling-promoted 1,2 hydride shift dynamics in vinyl to exhibit exponential growth with internal vibrational excitation, specifically achieving rates on the sub-μs time scale at energies above E ≈ 7500 cm-1. Most importantly, these results begin to elucidate the possible role of quantum isomerization through barriers without dissociation, in competition with the more conventional picture of classical roaming permitted over a much narrower window of energies immediately below the bond dissociation limit. Furthermore, when integrated over a Boltzmann distribution of thermal energies, these microcanonical tunneling rates are consistent with sub-μs time scales for 1,2 hydride shift dynamics at T > 1400 K. These results have potential relevance for combustion modeling of low-pressure flames, as well as recent observations of nuclear spin statistical mixing from high-resolution IR/microwave spectroscopy on vinyl radical. [ABSTRACT FROM AUTHOR]

Stimulated by recent experiments [B. E. Rocher-Casterline, L. C. Ch'ng, A. K. Mollner, and H. Reisler, J. Chem. Phys. 134, 211101 (2011)], we report quasiclassical trajectory calculations of the dissociation dynamics of the water dimer, (H2O)2 (and also (D2O)2) using a full-dimensional ab initio potential energy surface. The dissociation is initiated by exciting the H-bonded OH(OD)-stretch, as done experimentally for (H2O)2. Normal mode analysis of the fragment pairs is done and the correlated vibrational populations are obtained by (a) standard histogram binning (HB), (b) harmonic normal-mode energy-based Gaussian binning (GB), and (c) a modified version of (b) using accurate vibrational energies obtained in the Cartesian space. We show that HB allows opening quantum mechanically closed states, whereas GB, especially via (c), gives physically correct results. Dissociation of both (H2O)2 and (D2O)2 mainly produces either fragment in the bending excited (010) state. The H2O(J) and D2O(J) rotational distributions are similar, peaking at J = 3-5. The computations do not show significant difference between the ro-vibrational distributions of the donor and acceptor fragments. Diffusion Monte Carlo computations are performed for (D2O)2 providing an accurate zero-point energy of 7247 cm-1, and thus, a benchmark D0 of 1244 ± 5 cm-1. [ABSTRACT FROM AUTHOR]

We report rigorous calculations of rovibrational energies and dipole transition intensities for hydrogen peroxide using a new version of MULTIMODE as applied to molecules with torsional (reaction path) motion. The key features which permit such calculations for moderately sized polyatomic molecules of this general type are briefly described. A previous, accurate potential energy surface and a new high-level ab initio dipole moment surface are employed in these calculations. Detailed comparisons are made with high-resolution experimental spectral intensities from the HITRAN database. [ABSTRACT FROM AUTHOR]

The entrance channel potentials of the prototypical polyatomic reaction family X + CH4 → HX + CH3 (X = F, Cl, Br, I) are investigated using anion photoelectron spectroscopy and high-level ab initio electronic structure computations. The pre-reactive van der Waals (vdW) wells of these reactions are probed for X = Cl, Br, I by photodetachment spectra of the corresponding X--CH4 anion complex. For F-CH4, a spin-orbit splitting (∼1310 cm-1) much larger than that of the F atom (404 cm-1) was observed, in good agreement with theory. This showed that in the case of the F-CH4 system the vertical transition from the anion ground state to the neutral potentials accesses a region between the vdW valley and transition state of the early-barrier F + CH4 reaction. The doublet splittings observed in the other halogen complexes are close to the isolated atomic spin-orbit splittings, also in agreement with theory. [ABSTRACT FROM AUTHOR]

We employ recent flexible ab initio potential energy and dipole surfaces [Y. Wang, X. Huang, B. C. Shepler, B. J. Braams, and J. M. Bowman, J. Chem. Phys. 134, 094509 (2011)] to the calculation of IR spectra of the intramolecular modes of water clusters. We use a quantum approach that begins with a partitioned normal-mode analysis of perturbed monomers, and then obtains solutions of the corresponding Schrödinger equations for the fully coupled intramolecular modes of each perturbed monomer. For water clusters, these modes are the two stretches and the bend. This approach is tested against benchmark calculations for the water dimer and trimer and then applied to the water clusters (H2O)n for n = 6-10 and n = 20. Comparisons of the spectra are made with previous ab initio harmonic and empirical potential calculations and available experiments. [ABSTRACT FROM AUTHOR]

We present a full-dimensional potential energy surface and a dipole moment surface (DMS) for hydrated sodium ion. These surfaces are based on an n-body expansion for both the potential energy and the dipole moment, truncated at the two-body level for the H2O-Na+ interaction and also for the DMS. The water-water interaction is truncated at the three-body level. The new full-dimensional two-body H2O-Na+ potential is a fit to roughly 20 000 coupled-cluster single double (triple)/aug-cc-pVTZ energies. Properties of this two-body potential and the potential describing (H2O)nNa+ clusters, with n up to 4 are given. We then report anharmonic, coupled vibrational calculations with the 'local-monomer model' to obtain infrared spectra and also 0 K radial distribution functions for these clusters. Some comparisons are made with the recent infrared predissociation spectroscopy experiments of Miller and Lisy [J. Am. Chem. Soc. 130, 15381 (2008).] [ABSTRACT FROM AUTHOR]

We report full-dimensional, ab initio potential energy and dipole moment surfaces, denoted PES and DMS, respectively, for arbitrary numbers of water monomers. The PES is a sum of 1-, 2-, and 3-body potentials which can also be augmented by semiempirical long-range higher-body interactions. The 1-body potential is a spectroscopically accurate monomer potential, and the 2- and 3-body potentials are permutationally invariant fits to tens of thousands of CCSD(T)/aug-cc-pVTZ and MP2/aug-cc-pVTZ electronic energies, respectively. The DMS is a sum of 1- and 2-body DMS, which are covariant fits to tens of thousands MP2/aug-cc-pVTZ dipole moment data. We present the details of these new 2- and 3-body potentials and then extensive applications and tests of this PES are made to the structures, classical binding energies, and harmonic frequencies of water clusters up to the 22-mer. In addition, we report the dipole moment for these clusters at various minima and compare the results against available and new ab initio calculations. [ABSTRACT FROM AUTHOR]

We report full-dimensional, electronically adiabatic potential energy surfaces (PESs) for the ground state (1A′) and excited state (2A′) of OH3. The PESs are permutationally invariant fits to roughly 23 000 electronic energies (MRCI+Q/aVTZ). Classical trajectory calculations of the postquenching dynamics of OH A 2Σ+ are carried out on the 1A′ PES for H2 and D2, at previously identified conical intersections (CoIs) [B. C. Hoffman and D. R. Yarkony, J. Chem. Phys. 113, 10091 (2000)]. The initial momenta are sampled fully and partially microcanonically, corresponding to 'adiabatic' and 'diabatic' models of the dynamics, respectively. Branching ratios of reactive to nonreactive channels from separate C2v, C∞v, and Cs symmetries of CoIs are calculated, as are final rovibrational state distributions of OH and H2 products. The rovibrational distributions of the OH and D2 products, the D/H-atom translational energy distribution are calculated and compared to experimental ones. Agreement for these observable quantities is good. The branching between reactive and nonreactive quenching is sensitive to the momenta sampling; very good agreement with experiment is obtained using the diabatic sampling but not with the adiabatic sampling. The vibrational state distributions of H2O and HOD (although not measured by experiment) are also presented. [ABSTRACT FROM AUTHOR]

We report joint experimental and theoretical studies of outcomes resulting from the nonreactive quenching of electronically excited OD A 2Σ+ by H2. The experiments utilize a pump-probe technique to detect the OD X 2Π product state distribution under single collision conditions. The OD X 2Π products are observed primarily in their lowest vibrational state (v″=0) with substantially less population in v″=1. The OD X 2Π products are generated with a high degree of rotational excitation, peaking at N″=21 with an average rotational energy of 4600 cm-1, and a strong propensity for populating the Π(A′) Λ-doublet component indicative of alignment of the half-filled pπ orbital in the plane of OD rotation. Branching fraction measurements show that the nonreactive channel accounts for less than 20% of quenching outcomes. Complementary classical trajectory calculations of the postquenching dynamics are initiated from representative points along seams of conical intersections between the ground and excited-state potentials of OD(A 2Σ+,X 2Π)+H2. Diabatic modeling of the initial momenta in the dynamical calculations captures the key experimental trends: OD X 2Π products released primarily in their ground vibrational state with extensive rotational excitation and a branching ratio that strongly favors reactive quenching. The OD A 2Σ++H2 results are also compared with previous studies on the quenching of OH A 2Σ++H2; the two experimental studies show remarkably similar rotational energy distributions for the OH and OD X 2Π radical products. [ABSTRACT FROM AUTHOR]

Global analytic potential energy surfaces for O(3P)+H2O(1A1) collisions, including the OH+OH hydrogen abstraction and H+OOH hydrogen elimination channels, are presented. Ab initio electronic structure calculations were performed at the CASSCF+MP2 level with an O(4s3p2d1f)/H(3s2p) one electron basis set. Approximately 105 geometries were used to fit the three lowest triplet adiabatic states corresponding to the triply degenerate O(3P)+H2O(1A1) reactants. Transition state theory rate constant and total cross section calculations using classical trajectories to collision energies up to 120 kcal mol-1 (∼11 km s-1 collision velocity) were performed and show good agreement with experimental data. Flux-velocity contour maps are presented at selected energies for H2O collisional excitation, OH+OH, and H+OOH channels to further investigate the dynamics, especially the competition and distinct dynamics of the two reactive channels. There are large differences in the contributions of each of the triplet surfaces to the reactive channels, especially at higher energies. The present surfaces should support quantitative modeling of O(3P)+H2O(1A1) collision processes up to ∼150 kcal mol-1. [ABSTRACT FROM AUTHOR]

The effects of the reactant bending excitations in the F+CHD3 reaction are investigated by crossed molecular beam experiments and quasiclassical trajectory (QCT) calculations using a high-quality ab initio potential energy surface. The collision energy (Ec) dependence of the cross sections of the F+CHD3(vb=0,1) reactions for the correlated product pairs HF(v′)+CD3(v2=0,1) and DF(v′)+CHD2(v4=0,1) is obtained. Both experiment and theory show that the bending excitation activates the reaction at low Ec and begins to inactivate at higher Ec. The experimental F+CHD3(vb=1) excitation functions display surprising peak features, especially for the HF(v′=3)+CD3(v2=0,1) channels, indicating reactive resonances (quantum effects), which cannot be captured by quasiclassical calculations. The reactant state-specific QCT calculations predict that the v5(e) bending mode excitation is the most efficient to drive the reaction and the v6(e) and v5(e) modes enhance the DF and HF channels, respectively. [ABSTRACT FROM AUTHOR]

We report the properties of two novel transition states of the bimolecular hydrogen exchange reaction in the water dimer, based on an ab initio water dimer potential [A. Shank et al., J. Chem. Phys. 130, 144314 (2009)]. The realism of the two transition states is assessed by comparing structures, energies, and harmonic frequencies obtained from the potential energy surface and new high-level ab initio calculations. The rate constant for the exchange is obtained using conventional transition state theory with a tunneling correction. We employ a one-dimensional approach for the tunneling calculations using a relaxed potential from the full-dimensional potential in the imaginary-frequency normal mode of the saddle point, Qim. The accuracy of this one-dimensional approach has been shown for the ground-state tunneling splittings for H and D-transfer in malonaldehyde and for the D+H2 reaction [Y. Wang and J. M. Bowman, J. Chem. Phys. 129, 121103 (2008)]. This approach is applied to calculate the rate constant for the H2O+H2O exchange and also for H2O+D2O→2HOD. The local zero-point energy is also obtained using diffusion Monte Carlo calculations in the space of real-frequency-saddle-point normal modes, as a function of Qim. [ABSTRACT FROM AUTHOR]

Using full-dimensional EOM-IP-CCSD/aug-cc-pVTZ potential energy surfaces, the photoelectron spectrum, vibrational structure, and ionization dynamics of the water dimer radical cation, (H2O)2+, were computed. We also report an experimental photoelectron spectrum which is derived from photoionization efficiency measurements and compares favorably with the theoretical spectrum. The vibrational structure is also compared to the recent experimental work of Gardenier et al. [J. Phys. Chem. A 113, 4772 (2009)] and the recent theoretical calculations by Cheng et al. [J. Phys. Chem. A 113, 13779 (2009)]. A reduced-dimensionality nuclear Hamiltonian was used to compute the ionization dynamics for both the ground state and first excited state of the cation. The dynamics show markedly different behavior and spectroscopic signatures depending on which state of the cation is accessed by the ionization. Ionization to the ground state cation surface induces a hydrogen transfer which is complete within 50 fs, whereas ionization to the first excited state results in a much slower process. [ABSTRACT FROM AUTHOR]

We report the implementation of a previously suggested method to constrain a molecular system to have mode-specific vibrational energy greater than or equal to the zero-point energy in quasiclassical trajectory calculations [J. M. Bowman et al., J. Chem. Phys. 91, 2859 (1989); W. H. Miller et al., J. Chem. Phys. 91, 2863 (1989)]. The implementation is made practical by using a technique described recently [G. Czakó and J. M. Bowman, J. Chem. Phys. 131, 244302 (2009)], where a normal-mode analysis is performed during the course of a trajectory and which gives only real-valued frequencies. The method is applied to the water dimer, where its effectiveness is shown by computing mode energies as a function of integration time. Radial distribution functions are also calculated using constrained quasiclassical and standard classical molecular dynamics at low temperature and at 300 K and compared to rigorous quantum path integral calculations. [ABSTRACT FROM AUTHOR]

We report results of a classical trajectory calculation of the postquenching dynamics of OH A 2∑+ by H2. This is done by performing roughly 100 000 trajectories at previously identified conical intersections (CoIs) [B. C. Hoffman and D. R. Yarkony, J. Chem. Phys. 113, 10091 (2000)]. The initial momenta are sampled fully and partially microcanonically, corresponding to “adiabatic” and “diabatic” model of the dynamics, respectively. The trajectories are propagated on a new ground state ab initio-based potential energy surface. This surface is a permutationally invariant fit to roughly 23 000 electronic energies (multireference configuration interaction/correlation-consistent-augmented-triple-zeta basis) at configurations obtained mostly from direct-dynamics calculations (complete active space second order perturbation theory with correlation-consistent-augment double-zeta basis), also initiated at the CoIs. Final rovibrational state distributions of the ground electronic state OH product and the H-atom translational energy distributions for abstraction and insertion mechanisms are calculated and compared to experimental ones. Agreement for these observable quantities is good. The branching between reactive and nonreactive quenching is sensitive to the momenta sampling; very good agreement with experiment is obtained using the diabatic sampling but not with the adiabatic sampling. The calculated H2 rovibrational distributions (not measured experimentally) are also presented. [ABSTRACT FROM AUTHOR]

A quantum scattering dynamics, time-dependent wavepacket propagation method is applied to study the reaction of H2D++H2→H3++HD on the Xie–Braams–Bowman potential energy surface. The reduced-dimensional, seven-degree-of-freedom approach is employed in this calculation by fixing one Jacobi and one torsion angle related to H2D+ at the lowest saddle point geometry of D2d on the potential energy surface. Initial state selected reaction probabilities are presented for various initial rovibrational states. The ground state reaction probability shows no threshold for this reaction, in other words, this reaction can occur without an activation barrier. The vibrational excitation shows that the stretching motion of H+–HD only has a small effect on the reaction probability; the vibrational excitation of HD in H2D+ hinders the reactivity. By contrast, rotational excitation of H+–HD greatly enhances the reactivity with the reaction probability increased double or triple at high rotational states compared to the ground state. Reactive resonances, seen in all the initial state selected reaction probabilities, are also found in the integral cross section for the ground state of H2D+ and H2. The thermal rate coefficient is also calculated and is found to be in semiquantitative agreement with experiment; however, quantum scattering approaches including more degrees of freedom, especially including all the angles, are necessary to study this reaction in the future. [ABSTRACT FROM AUTHOR]

We report quasiclassical trajectory (QCT) calculations of the correlated product distributions and branching ratios of the reactions F+CHD3(v1=0,1)→HF(v)+CD3(v) and DF(v)+CHD2(v) using a recently published ab initio-based full-dimensional global potential energy surface [G. Czakó et al., J. Chem. Phys. 130, 084301 (2009)]. Harmonic normal mode analysis is done for the methyl products to determine the classical actions of each normal mode and then standard histogram binning and Gaussian binning (GB) methods are employed to obtain quantum state-resolved probabilities of the products. QCT calculations have been performed for both the vibrationally ground state and the CH stretching excited F+CHD3(v1=0,1) reactions at eight different collision energies in the 0.5–7.0 kcal/mol range. HF and DF vibrationally state-resolved rotational and angular distributions, CD3 and CHD2 mode-specific vibrational distributions, and correlated vibrationally state-specific distributions for the product pairs have been obtained and the correlated results were compared to the experiment. We find that the use of GB can be advantageous especially in the threshold regions. The CH stretching excitation in the reactant does not change the CD3 vibrational distributions significantly, whereas the HF molecules become vibrationally and rotationally hotter. On the other hand in the case of the DF+CHD2 channel the initially excited CH stretch appears mainly “intact” in the CHD2 product and the DF distributions are virtually the same as formed from the ground state CHD3 reaction. The computed results qualitatively agree with recent crossed molecular beam experiment [W. Zhang et al., Science 325, 303 (2009)] that (a) CHD2(v1=1) is the most populated product state of the F+CHD3(v1=1) reaction and this reaction produces much less CHD2(v=0) compared to the reaction F+CHD3(v=0); (b) the cross section ratio of CHD2(v1=1):CHD2(v=0) formed from the reactions F+CHD3(v1=1):F+CHD3(v=0) is less than 1 and shows little collision energy dependency; (c) the reactant CH stretch excitation increases the DF:HF ratio at low collision energies; (d) the correlated vibrational and angular distributions for DF(v)+CHD2(v1=0,1) from the ground state and stretch-excited reactions, respectively, are almost identical. [ABSTRACT FROM AUTHOR]

We report rigorous calculations of rovibrational energies and dipole transition intensities for three molecules using a new version of the code MULTIMODE. The key features of this code which permit, for the first time, such calculations for moderately sized but otherwise general polyatomic molecules are briefly described. Calculations for the triatomic molecule BF2 are done to validate the code. New calculations for H2CO and H2CS are reported; these make use of semiempirical potentials but ab initio dipole moment surfaces. The new dipole surface for H2CO is a full-dimensional fit to the dipole moment obtained with the coupled-cluster with single and double excitations and a perturbative treatment of triple excitations method with the augmented correlation consistent triple zeta basis set. Detailed comparisons are made with experimental results from a fit to relative data for H2CS and absolute intensities from the HITRAN database for H2CO. [ABSTRACT FROM AUTHOR]

We report full-dimensional, ab initio potential energy (PES) and dipole moment surfaces (DMS) for water. The PES is a sum of one-, two- and three-body terms. The three-body potential is a fit, reported here, to roughly 30,000 intrinsic three-body energies obtained with second-order Mo\ller–Plesset perturbation theory (MP2) and using the aug-cc-pVTZ basis set (avtz). The one- and two-body potentials are from an ab initio water dimer potential [Shank et al., J. Chem. Phys. 130, 144314 (2009)]. The predictive accuracy of the PES is demonstrated for the water trimer, tetramer, and hexamer by comparing the energies and harmonic frequencies obtained from the PES and new high level ab initio calculations at the respective global minima. The DMS is constructed from one- and two-body dipole moments, based on fits to MP2/avtz dipole moments. It is shown to be very accurate for the hexamer by comparison with direct calculations of the hexamer dipole. To illustrate the anharmonic character of the PES one-mode calculations of the 18 monomer fundamentals of the hexamer are reported in normal coordinates. [ABSTRACT FROM AUTHOR]