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Orbital‐optimized coupled‐cluster methods are very helpful for theoretical predictions of the molecular properties of challenging chemical systems, such as excited states. In this research, an efficient implementation of the equation‐of‐motion orbital‐optimized coupled‐cluster doubles method with the density‐fitting (DF) approach, denoted by DF‐EOM‐OCCD, is presented. The computational cost of the DF‐EOM‐OCCD method for excitation energies is compared with that of the conventional EOM‐OCCD method. Our results demonstrate that DF‐EOM‐OCCD excitation energies are dramatically accelerated compared to EOM‐OCCD. There are almost 17‐fold reductions for the C5H12 molecule in an aug‐cc‐pVTZ basis set with the RHF reference. This dramatic performance improvement comes from the reduced cost of integral transformation with the DF approach and the efficient evaluation of the particle‐particle ladder (PPL) term, which is the most expensive term to evaluate. Further, our results show that the DF‐EOM‐OCCD approach is very helpful for the computation of excitation energies in open‐shell molecular systems. Overall, we conclude that our new DF‐EOM‐OCCD implementation is very promising for the study of excited states in large‐sized challenging chemical systems. [ABSTRACT FROM AUTHOR]

In an effort to expand the existing QUEST database of accurate vertical transition energies [Véril et al. WIREs Comput. Mol. Sci.2021, 11, e1517], we have modeled more than 100 electronic excited states of different natures (local, charge‐transfer, Rydberg, singlet, and triplet) in a dozen of mono‐ and di‐substituted benzenes, including aniline, benzonitrile, chlorobenzene, fluorobenzene, nitrobenzene, among others. To establish theoretical best estimates for these vertical excitation energies, we have employed advanced coupled‐cluster methods including iterative triples (CC3 and CCSDT) and, when technically possible, iterative quadruples (CC4). These high‐level computational approaches provide a robust foundation for benchmarking a series of popular wave function methods. The evaluated methods all include contributions from double excitations (ADC(2), CC2, CCSD, CIS(D), EOM‐MP2, STEOM‐CCSD), along with schemes that also incorporate perturbative or iterative triples (ADC(3), CCSDR(3), CCSD(T)(a) ⋆, and CCSDT‐3). This systematic exploration not only broadens the scope of the QUEST database but also facilitates a rigorous assessment of different theoretical approaches in the framework of a homologous chemical series, offering valuable insights into the accuracy and reliability of these methods in such cases. We found that both ADC(2.5) and CCSDT‐3 can provide very consistent estimates, whereas among less expensive methods SCS‐CC2 is likely the most effective approach. Importantly, we show that some lower order methods may offer reasonable trends in the homologous series while providing quite large average errors, and vice versa. Consequently, benchmarking the accuracy of a model based solely on absolute transition energies may not be meaningful for applications involving a series of similar compounds. [ABSTRACT FROM AUTHOR]

Context: 1,3-Dithiole-2-thione-4,5-dithiolate (dmit) ligands are known for their conductive and optical properties. Dmit compounds have been assessed for use in sensor devices, information storage, spintronics, and optical material applications. Associations with various metallic centers endow dmit complexes with magnetic, optical, conductive, and antioxidant properties. Optical doping can facilitate the fabrication of magnetic conductor materials from ground-state nonmagnetic cations. While most studied complexes involve transition-metal centers due to their diverse chemistry, compounds with representative elements are less explored in the literature. This study investigated the structural and electronic properties of bisdmit complexes with representative Bi(III), Sb(III), and Zn(II) cations. AIMD calculations revealed two new geometries for Bi(III) and Zn(II) complexes, diverging from the isolated geometry typically used in quantum chemical calculations. The coordination of acetonitrile molecules to the cationic centers of the complexes resulted in unstable structures, while the dimerization of the complexes was stable. SA-CASSCF/NEVPT2 calculations were applied to the structures of the isolated complexes and stable dimers, confirming the multireference character of the electronic structure of the three systems and the multiconfigurational character of the Bi(III) complex. The electronic spectra simulated by the STEOM-DLPNO-CCSD calculations accurately reproduced the experimental UV‒Vis spectra indicating the participation of the isolated Bi(III) dmit complex and its dimeric form in solution. Methodology: AIMD calculations of the dmit salts were conducted using the GFN2-xTB method with 60 explicit acetonitrile molecules as the solvent at 300 K for a total simulation time of 50.0 ps, with printing intervals of 0.5 fs. The final geometries were optimized employing the PBEh-3c compound method, incorporating implicit conductor-like polarizable continuum model (CPCM) solvation for acetonitrile. Local energy decomposition (LED) analysis at the DLPNO-CCSD(T)/Def2-TZVP level of theory was utilized to investigate the stability of the complex geometries identified by AIMD. The electronic structures of the complexes were assessed using the SA-CASSCF/NEVPT2/Def2-TZVP method to confirm the multiconfigurational and multireference nature of their electronic structures. Electronic spectra were analyzed using the STEOM-DLPNO-CCSD/Def2-TZVP method, with CPCM used to simulate an acetonitrile medium. [ABSTRACT FROM AUTHOR]

A joint theoretical and experimental study on 32 endohedral silafullerane derivatives [X@Si20Y20]− (X=F‐I; Y=F‐I, H, Me, Et) and Th ${T_h }$ ‐[Cl@Si20H12Y8]− (Y=F‐I) is presented. First, we evaluated the structure‐determining template effect of Cl− in a systematic series of concave silapolyquinane model systems. Second, we investigated the X−→Si20 interaction energy (Eint ${E_{{\rm{int}}} }$) as a function of X− and Y and found the largest Eint ${E_{{\rm{int}}} }$ values for electron‐withdrawing exohedral substituents Y. Given that X− ions can be considered as Lewis bases and empty Si20Y20 clusters as Lewis acids, we classify our inseparable host–guest complexes [X@Si20Y20]− as "confined Lewis pairs". Third, 35Cl NMR spectroscopy proved to be highly diagnostic for an experimental assessment of the Cl−→Si20 interaction as the paramagnetic shielding and, in turn, δ ${\delta }$ (35Cl) of the endohedral Cl− ion correlate inversely with Eint ${E_{{\rm{int}}} }$. Finally, we disclose the synthesis of [PPN][Cl@Si20Y20] (Y=Me, Et, Br) and provide a thorough characterization of these new silafulleranes. [ABSTRACT FROM AUTHOR]

Fourteen highly reactive isomers of C5H and their ionic counterparts have been theoretically investigated using density functional theory (DFT) and coupled-cluster methods. The linear C5H (l-C5H) radical, pent-1,3-diyn-5-yliden-1-yl (1), along with its cationic form and the cyclic C5H (c-C5H), 1-ethynylcycloprop-1-en-2-yl-3-ylidene (2), have recently been detected in the Taurus Molecular Cloud-1. By using the UCCSD(T)/cc-pCVTZ level of theory, the calculated rotational constants and other spectroscopic parameters are found to be in good agreement with the available experimental data for isomers 1 and 2. Therefore, the current theoretical study may assist synthetic chemists and molecular spectroscopists in detecting other isomers in the laboratory or in the interstellar medium (ISM). Thermodynamically favorable rearrangement schemes for forming low-lying isomers 1, 2, and 3 have also been studied theoretically, and (2λ3-cycloprop-2-en-1-ylidene)ethenylidene (3) with a large dipole moment (μ = 4.73 Debye) is proposed to be a plausible candidate for detection in the ISM. [ABSTRACT FROM AUTHOR]

Auger decay is a relaxation process of core-vacant states in atoms and molecules, in which one valence electron fills the core vacancy while a second one is emitted. These states pose a challenge to electronic-structure theory, because they are embedded in the ionisation continuum. Recently, we showed that molecular Auger decay can be described using complex-variable coupled-cluster (CC) methods and that partial widths and branching ratios can be computed based on a decomposition of the CC energy. Here, we introduce channel-specific core-valence projectors, dubbed Auger channel projectors, as a more general technique to evaluate partial widths from complex-variable treatments. We apply this new method to core-ionised states of neon, water, ammonia and methane using CC singles and doubles (CCSD), equation-of-motion ionisation potential CCSD and configuration interaction singles (CIS) wave functions. Even though a single CIS calculation can never describe all Auger decay channels at once, we show that a combination of CIS calculations based on different reference states is able to recover partial and total decay widths from CC calculations to an excellent degree. [ABSTRACT FROM AUTHOR]

Fourteen highly reactive isomers of C5H and their ionic counterparts have been theoretically investigated using density functional theory (DFT) and coupled-cluster methods. The linear C5H (l-C5H) radical, pent-1,3-diyn-5-yliden-1-yl (1), along with its cationic form and the cyclic C5H (c-C5H), 1-ethynylcycloprop-1-en-2-yl-3-ylidene (2), have recently been detected in the Taurus Molecular Cloud-1. By using the UCCSD(T)/cc-pCVTZ level of theory, the calculated rotational constants and other spectroscopic parameters are found to be in good agreement with the available experimental data for isomers 1 and 2. Therefore, the current theoretical study may assist synthetic chemists and molecular spectroscopists in detecting other isomers in the laboratory or in the interstellar medium (ISM). Thermodynamically favorable rearrangement schemes for forming low-lying isomers 1, 2, and 3 have also been studied theoretically, and (2λ3-cycloprop-2-en-1-ylidene)ethenylidene (3) with a large dipole moment (μ = 4.73 Debye) is proposed to be a plausible candidate for detection in the ISM.

High‐level ab initio and density functional theory (DFT) calculations were performed to calculate the molecular properties of SbXn (X = F and Cl, n = 1–5) and SbXn− (X = F and Cl, n = 1–6), such as molecular structure, vibrational frequency, electron affinity (EA), and bond dissociation energy (BDE). The spin–orbit (SO) effect on the molecular properties was investigated using the SODFT method. The bond lengths optimized by the coupled‐cluster singles and doubles with perturbative triples method are in good agreement with the experimental values. The calculated vibrational frequencies agreed reasonably well with the available experimental values except for those of SbX4− (X = F and Cl) because the polar solvent affected the experimental results. SbX3 and SbX4− (X = F and Cl), whose Sb oxidation states are +3, exhibit substantial EAs and BDEs, respectively. The SO effect on SbX− and SbX2− (X = F and Cl) was considerable. [ABSTRACT FROM AUTHOR]

We introduce a variant of the multireference equation-of-motion coupled-cluster (MR-EOMCC) method where the amplitudes used for the similarity transformations are estimated from perturbation theory. Consequently, the new variant retains the many-body formalism, a reliance on at most two-body densities, and the state-universal character. As a non-iterative variant, computational costs are reduced, and no convergence difficulties with near-singular amplitudes can arise. Its performance was evaluated on several test sets covering transition metal atoms, small diatomics, and organic molecules against (near-)full CI quality reference data. We further highlight its efficacy on the weakly avoided crossing of LiF and place MR-EOMCC and the new variant into context with linear response theory. The accuracy of the variant was found to be at least on par with expectations for multireference perturbation theories, judging by the NEVPT2 method. The variant can be especially useful in multistate situations where the high accuracy of the iterative MR-EOMCC method is not required. [ABSTRACT FROM AUTHOR]

The reaction of H and CH2O, which plays an important role in combustion of hydrocarbons and alcohols, was studied in details using high-level coupled cluster calculations to construct a potential energy surface. This is followed by E,J-resolved master equation analyses to obtain rate constants as a function of both temperature and pressure. It is observed for the first time that the H-addition mechanism plays only a minor role (<10%), and can be neglected at high temperature in combustion environments. The direct H-abstraction pathway exclusively yields H2 plus CHO. The calculated rate constants from the first principles agree reasonably well with experiments for a wide range of temperatures between 250 and 3500 K. When there are no experimental data, we provide highly accurate rate constants for kinetics modelling. [ABSTRACT FROM AUTHOR]

A nearly linear scaling implementation of coupled‐cluster with singles and doubles excitations (CCSD) can be achieved by means of the domain‐based local pair natural orbital (DLPNO) method. The combination of DLPNO‐CCSD with implicit solvation methods allows the calculation of accurate energies and chemical properties of solvated systems at an affordable computational cost. We have efficiently implemented different schemes within the conductor‐like polarizable continuum model (C‐PCM) for DLPNO‐CCSD in the ORCA quantum chemistry suite. In our implementation, the overhead due to the additional solvent terms amounts to less than 5% of the time the equivalent gas phase job takes. Our results for organic neutrals and open‐shell ions in water show that for most systems, adding solvation terms to the coupled‐cluster amplitudes equations and to the energy leads to small changes in the total energy compared to only considering solvated orbitals and corrections to the reference energy. However, when the solute contains certain functional groups, such as carbonyl or nitrile groups, the changes in the energy are larger and estimated to be around 0.04 and 0.02 kcal/mol for each carbonyl and nitrile group in the solute, respectively. For solutes containing metals, the use of accurate CC/C‐PCM schemes is crucial to account for correlation solvation effects. Simultaneously, we have calculated the electrostatic component of the solvation energy for neutrals and ions in water for the different DLPNO‐CCSD/C‐PCM schemes. We observe negligible changes in the deviation between DLPNO‐CCSD and canonical‐CCSD data. Here, DLPNO‐CCSD results outperform those for Hartree‐Fock and density functional theory calculations. [ABSTRACT FROM AUTHOR]

We have performed a hierarchical ab initio benchmark and DFT performance study of D2Ch•••A− chalcogen bonds (Ch = S, Se; D, A = F, Cl). The ab initio benchmark study is based on a series of ZORA‐relativistic quantum chemical methods [HF, MP2, CCSD, CCSD(T)], and all‐electron relativistically contracted variants of Karlsruhe basis sets (ZORA‐def2‐SVP, ZORA‐def2‐TZVPP, ZORA‐def2‐QZVPP) with and without diffuse functions. The highest‐level ZORA‐CCSD(T)/ma‐ZORA‐def2‐QZVPP counterpoise‐corrected complexation energies (ΔECPC) are converged within 1.1–3.4 kcal mol−1 and 1.5–3.1 kcal mol−1 with respect to the method and basis set, respectively. Next, we used the ZORA‐CCSD(T)/ma‐ZORA‐def2‐QZVPP (ΔECPC) as reference data for analyzing the performance of 13 different ZORA‐relativistic DFT approaches in combination with the Slater‐type QZ4P basis set. We find that the three‐best performing functionals are M06‐2X, B3LYP, and M06, with mean absolute errors (MAE) of 4.1, 4.2, and 4.3 kcal mol−1, respectively. The MAE for BLYP‐D3(BJ) and PBE amount to 8.5 and 9.3 kcal mol−1, respectively. [ABSTRACT FROM AUTHOR]

We have performed a large‐scale evaluation of current computational methods, including conventional small‐molecule force fields; semiempirical, density functional, ab initio electronic structure methods; and current machine learning (ML) techniques to evaluate relative single‐point energies. Using up to 10 local minima geometries across ~700 molecules, each optimized by B3LYP‐D3BJ with single‐point DLPNO‐CCSD(T) triple‐zeta energies, we consider over 6500 single points to compare the correlation between different methods for both relative energies and ordered rankings of minima. We find that the current ML methods have potential and recommend methods at each tier of the accuracy‐time tradeoff, particularly the recent GFN2 semiempirical method, the B97‐3c density functional approximation, and RI‐MP2 for accurate conformer energies. The ANI family of ML methods shows promise, particularly the ANI‐1ccx variant trained in part on coupled‐cluster energies. Multiple methods suggest continued improvements should be expected in both performance and accuracy. [ABSTRACT FROM AUTHOR]

Alloy clusters of NaxLiy (4 ≤ x + y ≤ 10) are studied by exploring the potential energy surface in the ab initio MP2 level with the support of a quantum genetic algorithm (QGA). In some cases, the structures have been also refined with DFT and coupled-cluster methods. The general trends of sodium-lithium structures are in line with previous studies. The ionization potentials and polarizabilities to all structures were calculated with MP2 method and the average error between these two properties compared with experimental data was 6% and 13%, respectively. The topological analysis based on quantum theory of atoms in molecules (QTAIM) showed that by increasing the cluster size of the diatomic system there was a decrease of atomic interaction energies. The degree of degeneracy from D3BIA aromaticity index and the analysis of the atomic charges showed the influence (by charge transfer) of the chemical element in lower quantity in the cluster with respect to the other atoms. Our achievements of comparing our theoretical results with available experimental data have demonstrated that our approach can also predict satisfactorily quantum atomic and alloy clusters properties, at least, for low nuclearities. [ABSTRACT FROM AUTHOR]

The internally contracted multireference coupled-cluster theory (icMRCC) is reviewed, providing additional insights that we have gained over recent years. In addition, several modifications are presented that lead to an overall improvement in the method. These include a simpler and size extensive way to generate the excitation manifold and therefore determine the cluster amplitudes. Modifications and approximations are also introduced to the perturbative triples correction, icMRCCSD(T), which lead to a small improvement over the previous manifestation, at a reduced computational cost. With these reformulated methods, we can approach sub-chemical accuracy results. [ABSTRACT FROM AUTHOR]

Ionized water clusters, H2On+, have been of remarkable interest owing to their crucial roles in many chemical and biological processes. Small cationic water clusters H2On+, n = 2 to 6 serve as reasonable models for understanding the nature of the ionized water. In this study, employing high‐level ab initio quantum chemical methods, such as the density‐fitted orbital‐optimized linearized coupled‐cluster doubles (DF‐OLCCD), coupled‐cluster singles and doubles (CCSD), and coupled‐cluster singles and doubles with perturbative triples [CCSD(T)], a high‐accuracy study of structures and energetics for cationic water clusters [H2On+, n = 2‐6] is presented. In this study, 2 dimer, 8 trimer, 18 tetramer, 23 pentamer, and 25 hexamer clusters are reported. Most of the structures considered are reported for the first time. Relative, binding, and vertical attachment energies (VAEs), for the first time, are presented at the complete basis set (CBS) limit, extrapolating energies of the aug‐cc‐pVTZ and aug‐cc‐pVQZ basis sets, to provide the most accurate energetics to date. Our results demonstrate that as cluster size increases, the VAE value decreases, which indicates that large‐size clusters better compensate for the electron deficiency compared with small‐size clusters. The VAE values for pentamer and hexamer clusters are 118.5 to 165.5 and 121.9 to 153.7 kcal mol−1, respectively. Further, our binding energy results, at the CCSD(T)/CBS level, indicate strong bindings in cationic clusters due to hydrogen bond interactions. The average binding energy per water molecule varies from −16.6 to −21.8 kcal mol−1 for the clusters considered. Hence, we present the most extensive and accurate study on ionized water clusters to date. Further, our results indicate that the DF‐OLCCD method is very promising for ionic molecular clusters, and its accuracy approaches the CCSD(T) quality. The inexpensive analytic gradients of DF‐OLCCD, compared with CCSD(T), make it very helpful for high‐accuracy studies of molecular geometries. [ABSTRACT FROM AUTHOR]

Stabilisation technique is a traditional approach for studying resonances, such as the metastable autoionisation states. Stabilisation graphs are usually calculated by scaling the employed basis functions. Over the years, many methods for extracting the resonance complex energies from these graphs have been developed, nowadays, they are usually evaluated via analytical continuation schemes. However, many times it is difficult to obtain a well-behaved stabilisation graphs upon basis-set scaling. Herein, we illustrate that it is possible to shape stabilisation graphs to behave physically by adjusting the relevant Gaussian basis function exponents. We illustrate this for the excited Li–He and states [with He)], which serve as an excellent example since generating well-behaved graphs in this case is challenging. We show that by controlling and shaping the stabilisation graphs we can obtain converged complex energies, which are calculated by the resonances via Padé approach (a recently proposed analytical continuation scheme). Nevertheless, the concepts presented here are general to any stabilisation-graph based method. [ABSTRACT FROM AUTHOR]

A joint theoretical and experimental study on 32 endohedral silafullerane derivatives [X@Si20Y20]- (X- = F- -I- ; Y = F-I, H, Me, Et) and Th-[Cl@Si20H12Y8]- (Y = F-I) is presented. First, we evaluated the structure-determining template effect of Cl- in a systematic series of concave silapolyquinane model systems. Second, we investigated the X- →Si20 interaction energy (Eint) as a function of X- and Y and found the largest Eint values for electron-withdrawing exohedral substituents Y. Given that X- ions can be considered as Lewis bases and empty Si20Y20 clusters as Lewis acids, we classify our inseparable host-guest complexes [X@Si20Y20]- as "confined Lewis pairs". Third, 35Cl NMR spectroscopy proved to be highly diagnostic for an experimental assessment of the Cl- →Si20 interaction as the paramagnetic shielding and, in turn, δ(35Cl) of the endohedral Cl- ion correlate inversely with Eint. Finally, we disclose the synthesis of [PPN][Cl@Si20Y20] (Y = Me, Et, Br) and provide a thorough characterization of these new silafulleranes., (© 2023 Wiley-VCH GmbH.)

Abstract: The highly accurate ab initio equilibrium geometry of the hydrogen‐bonded uracil dimer is derived using a composite geometry extrapolation scheme based on all‐electron, complete basis set extrapolated Møller–Plesset perturbation theory using the jun‐pwCV[T,Q]Z basis sets combined with a valence CCSD(T)/cc‐pVTZ high‐level correction. Geometrical changes on dimerization are discussed and the performance of the several density functional approximations (among others SCAN, ωB97M‐V, DSD‐PBEP86‐D3(BJ), and DSD‐PBEP86‐NL) is evaluated. Orbital‐optimized MP2.5 is discussed as a reduced‐cost alternative to the CCSD(T) gradient in the composite scheme. A new reference interaction energy is calculated with explicitly correlated F12‐CCSD(T). [ABSTRACT FROM AUTHOR]

Partially linearized external models to active‐space coupled‐cluster through hextuple excitations, for example, CC{SDtqph}L, CCSD{tqph}L, and CCSD{tqph}hyb, are implemented and compared with the full active‐space CCSDtqph. The computational scaling of CCSDtqph coincides with that for the standard coupled‐cluster singles and doubles (CCSD), yet with a much large prefactor. The approximate schemes to linearize the external excitations higher than doubles are significantly cheaper than the full CCSDtqph model. These models are applied to investigate the bond dissociation energies of diatomic molecules (HF, F2, CuH, and CuF), and the potential energy surfaces of the bond dissociation processes of HF, CuH, H2O, and C2H4. Among the approximate models, CCSD{tqph}hyb provides very accurate descriptions compared with CCSDtqph for all of the tested systems. © 2018 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

We examine how geometry-dependent normalisation freedom of electronic wave-functions affects extraction of a meaningful diagonal Born-Oppenheimer correction (DBOC) to the ground-state Born-Oppenheimer potential energy surface (PES). By viewing this freedom as a kind of gauge-freedom, it is shown that DBOC and the resulting associated mass-dependent adiabatic PES are gauge-invariant quantities. A sum-over-states (SOS) formula for DBOC which explicitly exhibits this invariance is derived. A biorthogonal formulation suitable for DBOC computations using standard unnormalised coupled-cluster (CC) wave-functions is presented. This is shown to lead to a biorthogonal version of SOS formula with similar properties. On this basis, different computational schemes for evaluating DBOC using approximate CC wave-functions are derived. One of this agrees with the formula used in the current literature. The connection to adiabatic-to-diabatic transformations in non-adiabatic dynamics is explored and complications arising from biorthogonal nature of CC theory are identified. [ABSTRACT FROM AUTHOR]

A novel reduced-scaling, general-order coupled-cluster approach is formulated by exploiting hierarchical representations of many-body tensors, combined with the recently suggested formalism of scale-adaptive tensor algebra. Inspired by the hierarchical techniques from the renormalisation group approach, H/H2-matrix algebra and fast multipole method, the computational scaling reduction in our formalism is achieved via coarsening of quantum many-body interactions at larger interaction scales, thus imposing a hierarchical structure on many-body tensors of coupled-cluster theory. In our approach, the interaction scale can be defined on any appropriate Euclidean domain (spatial domain, momentum-space domain, energy domain, etc.). We show that the hierarchically resolved many-body tensors can reduce the storage requirements toO(N), whereNis the number of simulated quantum particles. Subsequently, we prove that any connected many-body diagram consisting of a finite number of arbitrary-order tensors, e.g. an arbitrary coupled-cluster diagram, can be evaluated inO(NlogN) floating-point operations. On top of that, we suggest an additional approximation to further reduce the computational complexity of higher order coupled-cluster equations, i.e. equations involving higher than double excitations, which otherwise would introduce a large prefactor into formalO(NlogN) scaling. [ABSTRACT FROM PUBLISHER]

The scalar–pseudoscalar interaction constant of PbF in its ground state electronic configuration is calculated using the Z-vector method in the relativistic coupled-cluster framework. The precise calculated value is very important to set upper bound limit on-odd scalar–pseudoscalar interaction constant,ks, from the experimentally observed-odd frequency shift. Further, the ratio of the effective electric field to the scalar–pseudoscalar interaction constant is also calculated which is required to get an independent upper bound limit of electric dipole moment of electron,de, andksand how these (deandks) are interrelated is also presented here. [ABSTRACT FROM PUBLISHER]

A coupled-cluster investigation of magnetic and electric properties of NF3, PF3and AsF3provides for a comparison with recent experimental data. For PF3, achieving reliable values for the magnetisability and rotationalg-tensor of PF3has been particularly challenging. We report the most accurate calculations to date for PF3; for the vibrationally corrected anisotropic magnetisability, our extrapolated CCSD(T)/CBS value of −0.290 a.u is within the uncertainty limits of the most recent experimental value of −0.286 ± 0.042 a.u. For the rotationalg-tensor of PF3, agreement between theory and experiment for theg⊥component is excellent (deviation of less than 0.0006 a.u.). However, the g||component remains problematic even though our vibrationally corrected CCSD(T)/CBS value of −0.0387 a.u is in closer agreement with the recently revised experimental value of −0.0470 ± 0.0020 a.u. than the original value of −0.0815 ± 0.0020 a.u. The origin of the remaining discrepancy remains unclear. Dipole and quadrupole moments have also been investigated. [ABSTRACT FROM PUBLISHER]

A new hardware-agnostic contraction algorithm for tensors of arbitrary symmetry and sparsity is presented. The algorithm is implemented as a stand-alone open-source code libxm. This code is also integrated with general tensor library libtensor and with the Q-Chem quantum-chemistry package. An overview of the algorithm, its implementation, and benchmarks are presented. Similarly to other tensor software, the algorithm exploits efficient matrix multiplication libraries and assumes that tensors are stored in a block-tensor form. The distinguishing features of the algorithm are: (i) efficient repackaging of the individual blocks into large matrices and back, which affords efficient graphics processing unit (GPU)-enabled calculations without modifications of higher-level codes; (ii) fully asynchronous data transfer between disk storage and fast memory. The algorithm enables canonical all-electron coupled-cluster and equation-of-motion coupled-cluster calculations with single and double substitutions (CCSD and EOM-CCSD) with over 1000 basis functions on a single quad-GPU machine. We show that the algorithm exhibits predicted theoretical scaling for canonical CCSD calculations, O( N6), irrespective of the data size on disk. © 2017 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

We examine the one-centre integrals used in semiempirical molecular orbital theory, for the elements H–Ne. The currently used parameters do not provide good estimates for the relative energies of ionised states of atoms. Directly calculating the one-electron integralsUssandUppwith coupled-cluster theory and fitting the two-electron repulsion integralsGssandGppto accurate coupled-cluster ionisation curves improves this behaviour. Since all the remaining parameters can be derived from these, the number of fitted variables is reduced from seven to two. The two-parameter model provides qualitative agreement with coupled-cluster theory for all ionisation potentials (IPs) and the principal electron affinity (EA). To obtain quantitative agreement for the principal IP and EA,UssandUppare included as variables in a four-parameter model. We discuss the new parameters and implications for the development of new, consistent semiempirical Hamiltonians. [ABSTRACT FROM PUBLISHER]

Quantum chemical calculations were performed to calculate the molecular properties of the 1 A g state of disulfur dinitride, S 2 N 2 , and the 1 A 1 state of the SN dimer, (SN) 2 , using density functional theory (DFT) and ab initio methods. The molecular structure of (SN) 2 is a trapezoid instead of a rectangle. Because the multireference character of (SN) 2 is considerable, most hybrid DFTs poorly describe its molecular properties. In contrast, old generalized gradient approximations give qualitatively correct descriptions of the molecular properties of (SN) 2 . Multi-state second-order multiconfigurational perturbation theory gives results that are close to those from multireference configuration interaction with the Davidson correction. The multireference character should be considered when calculating the molecular properties of poly sulfur nitride systems. [ABSTRACT FROM AUTHOR]

The molecular properties of bismuth monoboronyl, BiBO, were investigated using high-level ab initio and density functional theory calculations by including the effect of spin-orbit coupling (SOC). SOC does not cause any change in the BiB bond length of BiBO, by contrast it causes significant elongation of the BiB bond of BiBO−, by ∼0.03 Å. The BiB bond length of BiBO− that is calculated by considering SOC is almost identical to that of BiBO; this result is consistent with a recent experimental study. The term values of excited states of BiBO calculated by including SOC are in good agreement with the experimental results. One excited state which was not assigned in the previous experimental study is the Ω = 0+ state generated by strong SOC. In the theoretical calculations on molecules containing 6p-block elements, including SOC is crucial for obtaining results that are consistent with the corresponding experimental results. [ABSTRACT FROM AUTHOR]

We have performed a series of highly accurate calculations between CO 2 and the 20 naturally occurring amino acids for the investigation of the attractive noncovalent interactions. Different nucleophilic groups present in the amino acid structures were considered (α-NH 2 , COOH, side groups), and the stronger binding sites were identified. A database of accurate reference interactions energies was compiled as computed by explicitly-correlated coupled-cluster singles-and-doubles, together with perturbative triples extrapolated to the complete-basis-set limit. The CCSD(F12)(T)/CBS reference values were used for comparing a variety of popular density functionals with different basis sets. Our results show that most density functionals with the triple-zeta basis set def2-TZVPP align with the CCSD(F12)(T)/CBS reference values, but errors range from 0.1 kcal/mol up to 1.0 kcal/mol., (© 2023 Wiley-VCH GmbH.)

Titanium dioxide (TiO2) nanoparticles are widely used in contaminant remediation, photocatalysis and solar cell manufacturing. The low-cost production of TiO2 nanoparticles via the combustion of titanium tetrachloride (TiCl4) in oxygen is thus an important industrial process. To accurately model the flame synthesis of TiO2 nanoparticles, reliable thermodynamic data of Ti-O-Cl species are indispensable but often unavailable. We therefore carried out benchmark calculations, using the left-eigenstate completely renormalized singles, doubles and perturbative triples (CR-CC(2,3), aka CR-CCL) method with the cc-pVTZ basis set, to obtain the equilibrium structures and vibrational frequencies of selected Ti-O-Cl species; we then performed single-point CCSD(T)/aug-cc-pV LZ () calculations to extrapolate the CCSD(T)/CBS energies. After analyzing the experimental and calculated enthalpy of selected Ti-O-Cl species, the standard enthalpy of formation of the TiOCl2 molecule is determined to be 600.5kJ/mol at 298K. The standard enthalpy of all other Ti-O-Cl species are determined accordingly. Finally, we assessed the accuracy of 42 popular density functionals for the Ti-O-Cl species. Among these assessed functionals, the B98 functional, tightly followed by B97-1 and B3LYP, exhibits the best overall performance in the prediction of the thermochemistry of the Ti-O-Cl species. We conducted coupled-cluster benchmark calculations for the Ti-O-Cl species. The standard enthalpy and Gibbs energy of formation of 28 Ti-O-Cl species were determined at temperatures from 0 K to 3000 K. Among the 42 assessed density functionals, we recommend using B98, B97-1, and/or B3LYP functionals for the prediction of thermochemistry of the Ti-O-Cl system. [ABSTRACT FROM AUTHOR]

Peroxy radicals (RO2) are intermediates in fuel combustion, where they engage in efficiency-limiting autoignition reactions. They also participate in atmospheric chemistry leading to the formation of unwanted tropospheric ozone. Advances in spectroscopic techniques have allowed for the possibility of employing the lowest () electronic transition of RO2as a tool to selectively monitor these species, enabling accurate kinetic values to be obtained. Herein, high-levelab initiomethods are employed to systematically refine spectroscopic predictions for the methyl peroxy radical (CH3O2), one of the most abundant peroxy radicals in the atmosphere. In particular, vibrationally corrected geometries and anharmonic vibrational frequencies for both the ground () and first excited () state are predicted using coupled-cluster theory with up to perturbative triples [CCSD(T)] and large atomic natural orbital basis sets. Equation-of-motion coupled-cluster theory is utilised to compute verticaltransition properties; a radiative lifetime of 4.7 ms is suggested for the excited state. Finally, we predict the adiabatic excitation energy (T0) via systematic extrapolation to the complete basis limit of coupled-cluster with up to full quadruples (CCSDTQ). After accounting for several approximations, and including an anharmonic zero-point vibrational energy correction, we match experiment for this transition to within 9 cm−1. [ABSTRACT FROM AUTHOR]

As hydrogen-bonded systems are of utmost importance in especially biological and chemical systems, a new set of highly accurate reference dissociation energies, denoted HB49, is devised. For the molecules in this set, the basis set convergence of post-Hartree–Fock methods, including F12 methods, is investigated. Using combined Møller–Plesset perturbation theory (MP2) and CCSD(T) approaches for energies and MP2 and QCISD(T) for gradients, we achieve CCSD(T) accuracy, which has been determined before to yield an accuracy of 0.2 kJ/mol for a subset of HB49. Both conventional extrapolation techniques and F12 techniques are competitive with each other. By using MP2+ΔCCSD(T), a rather fast basis set convergence is obtained when both basis sets are carefully chosen. [ABSTRACT FROM PUBLISHER]

New algorithms for iterative diagonalization procedures that solve for a small set of eigen-states of a large matrix are described. The performance of the algorithms is illustrated by calculations of low and high-lying ionized and electronically excited states using equation-of-motion coupled-cluster methods with single and double substitutions (EOM-IP-CCSD and EOM-EE-CCSD). We present two algorithms suitable for calculating excited states that are close to a specified energy shift (interior eigenvalues). One solver is based on the Davidson algorithm, a diagonalization procedure commonly used in quantum-chemical calculations. The second is a recently developed solver, called the 'Generalized Preconditioned Locally Harmonic Residual (GPLHR) method.' We also present a modification of the Davidson procedure that allows one to solve for a specific transition. The details of the algorithms, their computational scaling, and memory requirements are described. The new algorithms are implemented within the EOM-CC suite of methods in the Q-Chem electronic structure program. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

A vibronic Hamiltonian and transition dipole operator parametrized by high-level equation-of-motion coupled-cluster calculations are used as a basis for predicting the dispersed fluorescence spectrum of the B ̃ − X ̃ origin band in the nitrate radical (NO 3). The position and intensities of transitions to ground state vibrational levels observed in the photodetachment spectrum of the NO 3 anion have previously been predicted to high fidelity with this Hamiltonian; it is expected that it also offers a basis for accurate prediction of the dispersed fluorescence spectrum. Comparison to experimental spectra provides considerable insight into various assignments. Significantly, this work joins a growing body of research that supports reassignment of the ground state ν 3 vibrational fundamental to the region between 1050–1060 cm−1, which is more than 400 cm−1 below the feature to which it has traditionally been assigned. The in-plane vibrational levels below 2000 cm−1 are reviewed, with theory and experiment agreeing to within 25 cm−1 for all that have been observed experimentally. Finally, the various vibronic coupling mechanisms that affect the ground state energy levels of NO 3 are summarized, and a discussion is provided about how the Lanczos algorithm can be used to efficiently calculate the dispersed fluorescence spectrum of the B ̃ − X ̃ origin band. [Display omitted] [ABSTRACT FROM AUTHOR]

Precise calculation of molecular electronic wavefunctions by methods such as coupled-cluster requires the computation of tensor contractions, the cost of which has polynomial computational scaling with respect to the system and basis set sizes. Each contraction may be executed via matrix multiplication on a properly ordered and structured tensor. However, data transpositions are often needed to reorder the tensors for each contraction. Writing and optimizing distributed-memory kernels for each transposition and contraction is tedious since the number of contractions scales combinatorially with the number of tensor indices. We present a distributed-memory numerical library (Cyclops Tensor Framework (CTF)) that automatically manages tensor blocking and redistribution to perform any user-specified contractions. CTF serves as the distributed-memory contraction engine in Aquarius, a new program designed for high-accuracy and massively-parallel quantum chemical computations. Aquarius implements a range of coupled-cluster and related methods such as CCSD and CCSDT by writing the equations on top of a C++ templated domain-specific language. This DSL calls CTF directly to manage the data and perform the contractions. Our CCSD and CCSDT implementations achieve high parallel scalability on the BlueGene/Q and Cray XC30 supercomputer architectures showing that accurate electronic structure calculations can be effectively carried out on top of general distributed-memory tensor primitives. [ABSTRACT FROM AUTHOR]