State-specific Coupled Cluster Method Research Articles

Perturbative triples correction to domain-based local pair natural orbital variants of Mukherjee's state specific coupled cluster method.

In this article we report an implementation of the perturbative triples correction to Mukherjee's state-specific multireference coupled cluster method based on the domain-based pair natural orbital approach (DLPNO-MkCC). We tested the performance of DLPNO-MkCCSD(T) in calculations involving tetramethyleneethane and isomers of naphthynes. These tests show that more than 97% of triples energy was recovered with respect to the canonical MkCCSD(T) method, which together with the DLPNO-MkCCSD part accounts for about 99.70-99.85% of the total correlation energy. The applicability of the method was demonstrated on calculations of singlet-triplet gaps for several large systems: triangulene, dynemicin A, and a beryllium complex.

Domain-Based Local Pair Natural Orbital Version of Mukherjee's State-Specific Coupled Cluster Method.

This article reports development of a local variant of Mukherjee's state-specific multireference coupled cluster method based on the domain-based pair natural orbital approach (DLPNO-MkCC). The current implementation is restricted to connected single and double excitations and model space with up to biexcited references. The performance of the DLPNO-MkCCSD was tested on calculations of tetramethyleneethane. The results show that above 99.9% of the correlation energy was recovered, with respect to the conventional MkCC method. To demonstrate the applicability of the method to large systems, singlet-triplet gaps of triangulene and bis(1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidine-2-ylidene)beryllium complex were studied. For the last system (105 atoms), we were able to perform a calculation in cc-pVTZ with 2158 basis functions on a single CPU in less than 9 days.

Perturbative triples correction for explicitly correlated Mukherjee’s state-specific coupled cluster method

This paper reports incorporation of the perturbative triples correction within the explicitly correlated Mukherjee’s multireference coupled cluster method using the SP ansatz. In accord with the standard approximation, these corrections are not directly entered by the correlation factor amplitudes, but the explicitly correlated part of the effective Hamiltonian is included in full. The performance of the new method is tested on singlet methylene, potential curve of fluorine molecule and automerisation barrier of cyclobutadiene. It has been found that the convergence pattern of the MkCCSD(T)-F12 results with increasing basis set is improved by approximately one cardinal number, as compared to conventional MkCCSD(T). This improvement appears at the level of single and double excitations, whereas no significant impact of the explicit treatment of the electron correlation on the (T) correction has been observed, in analogy to a single-reference approach.

An explicitly correlated Mukherjee's state specific coupled cluster method: development and pilot applications

This paper reports development of the explicitly correlated variant of Mukherjee's state specific multireference coupled cluster method (MkCC-F12). The current implementation is restricted to conventional single and double excitations and to pseudo-double excitations related to the Slater Type Geminal (STG) correlation factor using the SP ansatz. The performance of the MkCCSD-F12 was tested on calculations of singlet methylene, dissociation curve of the fluorine molecule, and the BeH(2) insertion pathway. As expected, the results of the newly developed method reconfirm the significantly faster convergence with respect to the basis set limit compared to the traditional expansion in Slater determinants. Results prove that treating the correlation factor separately for each reference is appropriate.

An ab initio determination of 1A 1- 3B 1 energy gap in CH 2 using orthogonally spin-adapted state-universal and state-specific coupled-cluster methods

The orthogonally spin-adapted open-shell state-universal (SU) and state-specific (SS) single and double coupled-cluster methods (CCSD) are applied to the singlet-triplet separation in methylene. Calculations have been performed using atomic-natural-orbital basis [5s4p3d2flg/3s2pld] and a few smaller basis sets. The best SS/SU CCSD result for the vibrationless 1A 1- 3B 1 separation ( T nr,BO e) using the nonrelativistic and Born-Oppenheimer approximations is 3315 cm −1. Accounting for the zero-point energies, relativistic and diagonal Born-Oppenheimer corrections yield T 0 = 3184 or 3133 cm −1, in excellent agreement with the experimental values of 3147 ± 5, 3156 ± 5 and 3165 ± 20 cm −1.

A study of 1A 1- 3B 1 separation in CH 2 using orthogonally spin-adapted state-universal and state-specific coupled-cluster methods

The orthogonally spin-adapted (OSA) state universal coupled-cluster method involving singly and doubly excited clusters (CCSD) and the recently proposed OSA open-shell state-specific CCSD method are applied to the problem of singlet-triplet separation in methylene. Both double zeta (DZ) and DZ plus polarization bases are utilized. Comparison with other correlation treatments, including limited and full configuration and various many-body approaches based on multi-configuration self-consistent-field reference, indicates that our method represents a highly efficient theoretical tool that can lead to reliable predictions of the 1A 1- 3B 1 splitting in CH 2 when larger bases are employed.