Search

The variational calculation of microwave or high-resolution rovibrational spectra is hampered by large memory requirements and long computation times. Simple sparsity techniques, the explicit account of symmetry, and different rotational basis functions have been employed to reduce these computational demands without deteriorating the final spectra. The dependence of the sparsity of the eigenvectors of the rovibrational Hamiltonian matrix with respect to the completeness of the Watson operator has been studied in detail. Benchmark calculations are provided for thioformaldehyde, thioformyl cyanide, and thiopropynal. [ABSTRACT FROM AUTHOR]

For floppy molecules showing internal rotations and/or large amplitude motions, curvilinear internal coordinates are known to be superior to rectilinear normal coordinates within vibrational structure calculations. Due to the myriad definitions of internal coordinates, automated and efficient potential energy surface generators necessitate a high degree of flexibility, supporting the properties arising from these coordinates. Within this work, an approach to deal with these challenges is presented, including key elements, such as the selection of appropriate fit functions, the exploitation of symmetry, the positioning of grid points, or elongation limits for different coordinates. These elements are tested for five definitions of curvilinear coordinates, with three of them being generated in an automated manner. Calculations for semi-rigid molecules, namely H2O, H2CO, CH2F2, and H2CNH, demonstrate the general functionality of the implemented algorithms. Additional calculations for the HOPO molecule highlight the benefits of these curvilinear coordinates for systems with large amplitude motions. This new implementation allowed us to compare the performance of these different coordinate systems with respect to the convergence of the underlying expansion of the potential energy surface and subsequent vibrational configuration interaction calculations. [ABSTRACT FROM AUTHOR]

While the vibrational spectra of semi-rigid molecules can be computed on approaches relying on the Watson Hamiltonian, floppy molecules or molecular clusters are better described by Hamiltonians, which are capable of dealing with any curvilinear coordinates. It is the kinetic energy operator (KEO) of these Hamiltonians, which render the correlated calculations relying on them rather costly. Novel implementation of vibrational self-consistent field theory and vibrational configuration interaction theory on the basis of the Podolsky Hamiltonian are reported, in which the inverse of the metric tensor, i.e., the G matrix, is represented by an n-mode expansion expressed in terms of polynomials. An analysis of the importance of the individual terms of the KEO with respect to the truncation orders of the n-mode expansion is provided. Benchmark calculations have been performed for the cis-HOPO and methanimine, H2CNH, molecules and are compared to experimental data and to calculations based on the Watson Hamiltonian and the internal coordinate path Hamiltonian. [ABSTRACT FROM AUTHOR]

The positions of grid points for representing a multidimensional potential energy surface (PES) have a non-negligible impact on its accuracy and the associated computational effort for its generation. Six different positioning schemes were studied for PESs represented by n-mode expansions as needed for the accurate calculation of anharmonic vibrational frequencies by means of vibrational configuration interaction theory. A static approach, which has successfully been used in many applications, and five adaptive schemes based on Gaussian process regression have been investigated with respect to the number of necessary grid points and the accuracy of the fundamental modes for a small set of test molecules. A comparison with a related, more sophisticated, and consistent approach by Christiansen et al. is provided. The impact of the positions of the ab initio grid points is discussed for multilevel PESs, for which the computational effort of the individual electronic structure calculations decreases for increasing orders of the n-mode expansion. As a result of that, the ultimate goal is not the maximal reduction of grid points but rather the computational cost, which is not directly related. [ABSTRACT FROM AUTHOR]

Thioformyl cyanide, HC(S)CN, and its deuterated isotopologue have been studied by high-level ab initio methods. Based on multidimensional potential energy surfaces including up to four-mode coupling terms and obtained from explicitly correlated coupled-cluster calculations, their geometrical parameters, vibrational transitions, rovibrational spectrum and spectroscopic constants have been studied in detail. The latter two properties, being obtained from vibrational configuration interaction theory (VCI) and vibrational perturbation theory (VPT), have been compared with available experimental data. The rotational and rovibrational spectra have been studied by rovibrational configuration interaction theory (RVCI) and colour coding has been used to obtain detailed insight into the rotational fine structure. [ABSTRACT FROM AUTHOR]

Rotational and rovibrational spectra are a key in astrophysical studies, atmospheric science, pollution monitoring, and other fields of active research. The ab initio calculation of such spectra is fairly sensitive with respect to a multitude of parameters and all of them must be carefully monitored in order to yield reliable results. Besides the most obvious ones, i.e., the quality of the multidimensional potential energy surface and the vibrational wavefunctions, it is the representation of the μ-tensor within the Watson Hamiltonian, which has a significant impact on the desired line lists or simulated spectra. Within this work, we studied the dependence of high-resolution rovibrational spectra with respect to the truncation order of the μ-tensor within the rotational contribution and the Coriolis coupling operator of the Watson operator. Moreover, the dependence of the infrared intensities of the rovibrational transitions on an n-mode expansion of the dipole moment surface has been investigated as well. Benchmark calculations are provided for thioformaldehyde, which has already served as a test molecule in other studies and whose rovibrational spectrum was found to be fairly sensitive. All calculations rely on rovibrational configuration interaction theory and the discussed high-order terms of the μ-tensor are a newly implemented feature, whose theoretical basics are briefly discussed. [ABSTRACT FROM AUTHOR]

Rotational constants and centrifugal distortion constants of a molecule are the essence of its rotational or rovibrational spectrum (e.g., from microwave, millimeter wave, and infrared experiments). These parameters condense the spectroscopic characteristics of a molecule and, thus, are a valuable resource in terms of presenting and communicating spectroscopic observations. While spectroscopic parameters are obtained from experimental spectra by fitting an effective rovibrational Hamiltonian to transition frequencies, the ab initio calculation of these parameters is usually done within vibrational perturbation theory. In the present work, we investigate an approach related to the experimental fitting procedure, but relying solely on ab initio data obtained from variational calculations, i.e., we perform a nonlinear least squares fit of Watson's A- and S-reduced rotation–vibration Hamiltonian to rovibrational state energies (resp. transition frequencies) from rotational–vibrational configuration interaction calculations. We include up to sextic centrifugal distortion constants. By relying on an educated guess of spectroscopic parameters from vibrational configuration interaction and vibrational perturbation theory, the fitting procedure is very efficient. We observe excellent agreement with experimentally derived parameters. [ABSTRACT FROM AUTHOR]

Within incremental vibrational configuration interaction theory (iVCI), the vibrational state energy is determined by means of a many-body expansion, i.e., it is a sum of terms of increasing order, which allow for an embarrassingly parallel evaluation. The convergence of this expansion depends strongly on the definition of the underlying bodies, which essentially decompose the correlation space into fragments. The different definitions considered here comprise mode-based bodies, excitation level-based bodies, and energy-based bodies. An analysis of the convergence behavior revealed that accounting for resonances within these definitions is mandatory and leads to a substantial improvement of the convergence, that is, the expansions can be truncated at lower orders. Benchmark calculations and systematic comparisons of the different body definitions for a small set of molecules, i.e., ketene, ethene, and diborane, have been conducted to study the overall performance of these iVCI implementations with respect to accuracy and central processing unit time. [ABSTRACT FROM AUTHOR]

Rovibrational configuration interaction theory has been used to study the rotational spectrum and low lying rovibrational states of thiopropynal, a molecule, which has recently been detected in an interstellar cloud. Geometrical parameters, fundamental vibrational transitions and spectroscopic constants are also provided. All calculations rely on an n$$ n $$‐mode representation of the multidimensional potential energy surface, which has been obtained from explicitly correlated coupled‐cluster approaches including core correlation effects and an approximate treatment of high‐order coupled‐cluster terms. Comparison with experimental data is provided for several properties and the agreement was found to be excellent in most cases. [ABSTRACT FROM AUTHOR]

An outline of a newly developed program for the simulation of rovibrational nonresonant Raman spectra is presented. This program is an extension of our recently developed code for rovibrational infrared spectra [Erfort et al., J. Chem Phys. 152, 244104 (2020)] and relies on vibrational wavefunctions from variational configuration interaction theory to allow for an almost fully automated calculation of such spectra in a pure ab initio fashion. Due to efficient contraction schemes, this program requires modest computational resources, and it can be controlled by only a few lines of input. As the required polarizability surfaces are also computed in an automated fashion, this implementation enables the routine application to small molecules. For demonstrating its capabilities, benchmark calculations for water H216O are compared to reference data, and spectra for the beryllium dihydride dimer, Be2H4 (D2h), are predicted. The inversion symmetry of the D2h systems lead to complementary infrared and Raman spectra, which are both needed for a comprehensive investigation of this system. [ABSTRACT FROM AUTHOR]

The implementation of an algorithm for the determination of vibrational state energies based on a many-body expansion within the framework of configuration interaction theory is presented. An efficient evaluation of the increments within this approach is realized by an iterative configuration selection scheme. The new algorithm is characterized by low memory demands and an embarassingly parallel workload. The convergence of the expansion has been studied for a series of small molecules of increasing size, namely, formaldehyde, ketene, ethylene, and diborane. A threshold function has been employed to reduce the number of increments for high orders of the expansion. Benchmark calculations with respect to customary configuration-selective vibrational configuration interaction calculations are provided. [ABSTRACT FROM AUTHOR]

The implementation of a new program for the variational calculation of rovibrational state energies and infrared intensities is presented. The program relies on vibrational self-consistent field and vibrational configuration interaction theory and is based on the Watson Hamiltonian. All needed prerequisites, i.e., multidimensional potential energy and dipole moment surfaces, comprehensive symmetry information, the determination of vibrational wave functions, and an efficient calculation of partition functions, are computed in a fully automated manner, which allows us to calculate rovibrational spectra in a black-box type fashion. Moreover, the use of a molecule specific rotational basis leads to reliable rovibrational line lists. Benchmark calculations are provided for thioformaldehyde (H2CS), which shows strong Coriolis coupling effects and a complex rovibrational spectrum. The underlying multidimensional potential energy surface has been calculated at the level of explicitly correlated coupled-cluster theory. [ABSTRACT FROM AUTHOR]

The assignment of vibrational states is an integral part of quantum chemical calculations, which supports the analysis of experimental infrared spectra. In variational calculations, usually, it is the leading coefficient of the configuration interaction vector, which provides the state identity. However, this concept will possibly fail in case of special coordinate systems, such as, for example, localized normal coordinates, or within calculations for overtones of non-Abelian molecules, when a real valued configuration basis has been employed. A combination of both renders a proper assignment fairly tedious. We present a route to overcome this problem by using a highly efficient calculation of multidimensional overlap integrals based on the Smolyak quadrature. Beside this, a general protocol for the symmetry assignment of vibrational states will be discussed, which completes a general assignment. Extensive benchmark calculations are provided for the fundamental modes and overtones of chloromethane, CH3Cl, in canonical and localized normal coordinates based on accurate potential energy surfaces obtained from explicitly correlated coupled-cluster theory. In addition, the linear CNNC molecule has been studied, for which hardly any reference data do exist. [ABSTRACT FROM AUTHOR]

The rovibrational spectra of metaphosphorous acid, HOPO, and its deuterated isotopologue have been studied by vibrational configuration interaction calculations, relying on the internal coordinate path Hamiltonian and the Watson Hamiltonian. Tunneling effects for the overtones of the torsional mode, which gives rise to the cis–trans isomerization, and its rovibrational transitions have been investigated in detail. Due to strong matrix effects, comparison with experimental data is hindered, and thus, the calculations provide accurate estimates for the fundamental modes of these species. [ABSTRACT FROM AUTHOR]

Molpro is a general purpose quantum chemistry software package with a long development history. It was originally focused on accurate wavefunction calculations for small molecules but now has many additional distinctive capabilities that include, inter alia, local correlation approximations combined with explicit correlation, highly efficient implementations of single-reference correlation methods, robust and efficient multireference methods for large molecules, projection embedding, and anharmonic vibrational spectra. In addition to conventional input-file specification of calculations, Molpro calculations can now be specified and analyzed via a new graphical user interface and through a Python framework. [ABSTRACT FROM AUTHOR]

The new 6π‐electron four‐membered ring compound 3‐fluoro‐1λ2,2,4,3λ3‐thiadiazaphosphetidine, FP(μ‐N)2S, has been generated in the gas phase through high‐vacuum flash pyrolysis (HVFP) of thiophosphoryl diazide, FP(S)(N3)2, at 1000 K. Subsequent isolation of FP(μ‐N)2S in cryogenic matrices (Ar, Ne, and N2) allows its characterization with matrix‐isolation IR and UV‐vis spectroscopy by combination with 15N‐isotope labeling and computations at the CCSD(T)‐F12a/VTZ‐F12 level of theory. Upon visible‐light irradiation at 550 nm, this cyclic compound undergoes ring‐opening to the thiazyl isomer FPNSN, followed by dissociation to FP and SN2 under subsequent UV‐irradiation at 365 nm. In sharp contrast to the square planar structure for the isolobal four‐membered ring S2N2, a puckered structure with significant biradical character has been found for FP(μ‐N)2S. [ABSTRACT FROM AUTHOR]

The time-independent eigenstate-free Raman wavefunction approach for calculating anharmonic vibronic spectra has been extended for the calculation of Herzberg–Teller contributions on the basis of an n-mode expansion of the transition electric dipole moment surface. This allows for the efficient simulation of Franck–Condon dark vibronic spectra. In addition, vibrational angular momentum terms have been implemented into this formalism, as they are important for an accurate description of vibrational wavefunctions spanning double-well potentials. This approach has been used to compute the FC-forbidden vibronic spectrum of the n → π* transition of formaldehyde based on a potential energy and transition dipole moment surfaces obtained from explicitly correlated multi-reference configuration interaction calculations. An extensive analysis of the resulting vibronic structure is provided, which allows for a detailed assignment and interpretation of the experimental spectrum. [ABSTRACT FROM AUTHOR]

The vibrational spectra of simple C H 3 + —Rg (Rg = He, Ne, Ar, Kr) complexes have been studied by vibrational configuration interaction theory relying on multidimensional potential energy surfaces (PESs) obtained from explicitly correlated coupled cluster calculations, CCSD(T)-F12a. In agreement with experimental results, the series of rare gas atoms leads to rather unsystematic results and indicates huge zero point vibrational energy effects for the helium complex. In order to study these sensitive complexes more consistently, we also introduce configuration averaged vibrational self-consistent field theory, which is a generalization of standard vibrational self-consistent field theory to several configurations. The vibrational spectra of the complexes are compared to that of the methyl cation, for which corrections due to scalar-relativistic effects, high-order coupled-cluster terms, e.g., quadruple excitations, and core-valence correlation have explicitly been accounted for. The occurrence of tunneling splittings for the vibrational ground-state of C H 3 + —He has been investigated on the basis of semiclassical instanton theory. These calculations and a direct comparison of the energy profiles along the intrinsic reaction coordinates with that of the hydronium cation, H3O+, suggest that tunneling effects for vibrationally excited states should be very small. [ABSTRACT FROM AUTHOR]

The simplest diphosphene HPPH and isomeric diphosphinyldene PPH2 features prototype phosphorus‐phosphorus multiple bonding properties that have been of long‐standing interest in main‐group chemistry. Herein, we report the observation of cis‐HPPH, trans‐HPPH, and PPH2 among the respective laser photolysis products of phosphine (PH3) and diphosphine (P2H4) in solid N2‐ and Ar‐matrices at 10 K. The identification of these P2H2 isomers with matrix‐isolation IR and UV/Vis spectroscopy is supported by D‐isotope labeling and the quantum chemical calculations at the CCSD(T)‐F12a/cc‐pVTZ‐F12 level using configuration‐selective vibrational configuration interaction theory (VCI). Bonding analyses suggest that the two conformers of HPPH contain standard PP double bonds, whereas, PPH2 resembles P2 in having partial PP triple bond due to the H2P←P π bonding interaction. [ABSTRACT FROM AUTHOR]

The simplest diphosphene HPPH and isomeric diphosphinyldene PPH2 features prototype phosphorus‐phosphorus multiple bonding properties that have been of long‐standing interest in main‐group chemistry. Herein, we report the observation of cis‐HPPH, trans‐HPPH, and PPH2 among the respective laser photolysis products of phosphine (PH3) and diphosphine (P2H4) in solid N2‐ and Ar‐matrices at 10 K. The identification of these P2H2 isomers with matrix‐isolation IR and UV/Vis spectroscopy is supported by D‐isotope labeling and the quantum chemical calculations at the CCSD(T)‐F12a/cc‐pVTZ‐F12 level using configuration‐selective vibrational configuration interaction theory (VCI). Bonding analyses suggest that the two conformers of HPPH contain standard PP double bonds, whereas, PPH2 resembles P2 in having partial PP triple bond due to the H2P←P π bonding interaction. [ABSTRACT FROM AUTHOR]

Aminoborane, H2NBH2 and its isotopologues, H2N10BH2, D2NBD2, and D2N10BD2, have been studied by high‐level ab initio methods. All calculations rely on multidimensional potential energy surfaces and dipole moment surfaces including high‐order mode coupling terms, which have been obtained from electronic structure calculations at the level of explicitly correlated coupled‐cluster theory, CCSD(T)‐F12, or the distinguishable cluster approximation, DCSD. Subsequent vibrational structure calculations based on second‐order vibrational perturbation theory, VPT2, and vibrational configuration interaction theory, VCI, were used to determine rotational constants, centrifugal distortion constants, vibrationally averaged geometrical parameters and (an)harmonic vibrational frequencies. The impact of core‐correlation effects is discussed in detail. Rovibrational VCI calculations were used to simulate the gas phase spectra of these species and an in‐depth analysis of the ν7 band of aminoborane is provided. Color‐coding is used to reveal the identity of the individual progressions of the rovibrational transitions for this particular mode. [ABSTRACT FROM AUTHOR]

A method is presented, which allows for the rigorous use of symmetry within the construction of multidimensional potential energy surfaces (PESs). This approach is based on a crude but very fast energy estimate, which retains the symmetry of a molecule. This enables the efficient use of coordinate systems, which mix molecular and permutational symmetry, as, for example, in the case of normal coordinates with subsets of localized normal coordinates. The impact of symmetry within the individual terms of an expansion of the PES is studied together with a symmetry consideration within the individual electronic structure calculations. A trade between symmetry within the surface and the electronic structure calculations has been observed and has been investigated in dependence on different coordinate systems. Differences occur between molecules belonging to Abelian point groups in contrast to non-Abelian groups, in which further benefits can be achieved by rotating normal coordinates belonging to degenerate vibrational frequencies. In general, the exploitation of surface symmetry was found to be very important within the construction of PESs of small and medium-sized molecules—irrespective of the coordinate system. Benchmark calculations are provided for formaldehyde, ethene, chloromethane, and cubane. [ABSTRACT FROM AUTHOR]