Spin–orbit coupling and electron correlation in relativistic configuration interaction and coupled-cluster methods

• Published in: Chemical physics Vol. 395; pp. 115 - 121
• Main Authors: Kim, Inkoo, Park, Young Choon, Kim, Hyungjun, Lee, Yoon Sup
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 20-02-2012
• Subjects: Configuration interaction; Coupled-cluster method; Electron correlation; Spin–orbit coupling; Two-component calculation; Two-component calculation; Spin–orbit coupling; Coupled-cluster method; Electron correlation; Configuration interaction
• ISSN: 0301-0104
• DOI: 10.1016/j.chemphys.2011.05.002

Spinor-based correlated methods are reliable in most cases, but coupled-cluster methods are very effective in recovering spin-orbit effects even for orbital based references and from the single excitation level. [Display omitted]
► Performances of Kramers restricted (KR) and spin–orbit (SO), CI and CC are compared. ► KR-CI and CC exhibit good qualities with respect to increase in spin–orbit coupling. ► SOCI requires triple or quadruple excitations to generate reasonable results. ► SOCC shows a negligible dependence on the strength of spin–orbit coupling. ► Success of SOCC is due to the effective recovery of the spin–orbit energy by SOCCS.
We studied convergence characteristics of relativistic effective core potential (RECP) based configuration interaction (CI) and coupled-cluster (CC) schemes in terms of spin–orbit coupling and electron correlation. The relativistic correlated methods can be divided into Kramers restricted (KR) and spin–orbit (SO) methods which differ by the stage of spin–orbit treatment: the KR method employs two-component Kramers restricted Hartree–Fock (HF) spinors as the one-electron basis in which spin–orbit coupling is included, whereas the SO method is based on one-component molecular orbitals generated from scalar relativistic HF and the spin–orbit interaction is then entered in post-HF step. The KR method is usually superior to the SO method for molecules containing heavy elements since spin–orbit coupling is included from the HF step. A performance calibration of the SO method against the KR method is performed by computations of the ground state energies and equilibrium bond lengths of MH (M=Tl, Pb, Bi, Po, and At). Spin–orbit coupling of each molecule was systematically increased by adjusting the spin–orbit operator of RECP to investigate its impact on the SO method. Although KRCI and SOCI converged to the same full-CI limit, for the strong spin–orbit coupling SOCI required higher levels of correlation compared to KRCI to account for the orbital relaxation effect. SOCC, in contrast, was able to recover both spin–orbit interaction and electron correlation in CC steps regardless of the spin–orbit strength, implying that SOCC could be the reliable and efficient relativistic ab initio method for moderate sized molecules containing heavy elements.