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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]

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]

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]

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]

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]

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]

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]

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]

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]

In this work, we use MP2 and coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)] as well as their corresponding explicitly correlated (F12) counterparts to compute the interaction energies of water icosamers. The incremental scheme is used to compute benchmark energies at the CCSD(T)/CBS(45) and CCSD(T)(F12*)/cc-pVQZ-F12 level of theory. The four structures, dodecahedron, edge sharing, face sharing, and fused cubes, are part of the WATER27 test set and therefore, highly accurate interaction energies are required. All methods applied in this work lead to new benchmark energies for these four systems. To obtain these values, we carefully analyze the convergence of the interaction energies with respect to the basis set. Furthermore, we investigate the influence of the basis set superposition error and the core-valence correlation. The interaction energies are: dodecahedron −198.6 kcal/mol, edge sharing −209.7 kcal/mol, face sharing −208.0 kcal/mol, and fused cubes −208.0 kcal/mol. For water clusters, we recommend to use the PW6B95 density functional of Truhlar in combination with Grimme's dispersion correction (D3), as the mean absolute error is 0.9 and the root mean-squared deviation is only 1.4 kcal/mol. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

In this work, we report a theoretical investigation concerning the use of the popular coupled-cluster//Kohn-Sham density functional theory (CC//KS-DFT) model chemistry, here applied to study the entrance channel of the [ABSTRACT FROM AUTHOR]

DFT methods are used in calculations of geometries, energies, and frequencies for O (3P) + CH3OH reactions. CCSD(T) single-point calculations are carried out followed by extrapolation to the complete basis set (CBS) limit and inclusion of core correlation. The best estimate results of the reaction enthalpies (ΔH00) for the first (-7.1 kcal/mol), second (2.0 kcal/mol), and the third (25.8 kcal/mol) reactions path are in excellent agreement with the experimental values, i.e., -7.2 ± 0.2, 1.2 ± 1.0, and 26.0 ± 1.2 kcal/mol, respectively. Rate constants and activation energies in the range of 300–2,000 K calculated with the variational transition state theory are in good agreement with previous studies. H/D and 12C/13C kinetic isotope effects are also calculated in good agreement with available experimental data. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

My collaborations with Frank Harris are reviewed. Besides reminiscing commentary on our long association, I explain the essentials of a recent proposal by Frank and me for a correlated basis set to implement the Molecular Coupled Cluster method I published in 1987. Its use will require the accurate evaluation of integrals over fully correlated Gaussian-type n body functions. This has been achieved for arbitrary n in a pair of publications. A third paper by Frank shows the way to include angular factors. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

Accurate equilibrium geometries, harmonic vibrational frequencies, and equilibrium rotational constants of the methyl radical were computed using coupled-cluster theory in combination with the cc-pVnZ and cc-pCVnZ basis sets of Dunning. Valence and all electron calculations were carried out. Using all electron correlations and CBS extrapolation, the re value is equal to 1.0758 Å, which is 0.0032 Å shorter than the experimental value of r0 (1.079 Å). The harmonic (ω1, ω2, ω3, and ω4) frequencies calculated using the CBS extrapolation differ from the experimental fundamental frequencies, respectively, by +142, +125, +21, and –82 cm-1. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [ABSTRACT FROM AUTHOR]