ABSTRACT
Spin–orbit coupling (SOC) in EOM-CCSD calculations can be treated in SCF step (SOC-EOM-CCSD), in both the CCSD and EOM steps (EOM-CCSD(SOC)), only in the EOM step (EOM(SOC)-CCSD) or perturbatively (pSOC-EOM-CCSD). In this work, we report implementation of pSOC-EOM-CCSD based on the EOM(SOC)-CCSD program by directly calculating the SOC matrix between two states without using the transition density matrix. Computational cost of pSOC-EOM-CCSD can be further reduced particularly for excited states by using only the r-amplitudes (rpSOC-EOM-CCSD). Accuracy of EOM-CCSD(SOC) is evaluated by comparing its results with those of SOC-EOM-CCSD for heavy and superheavy elements. Highly accurate results are obtained with EOM-CCSD(SOC) for elements up to the sixth-row, and reasonable results are achieved for superheavy elements when contribution of the 7p1/2 spinor to the involved occupied orbitals is insignificant. Performance of EOM(SOC)-CCSD, and (r)pSOC-EOM-CCSD is investigated by comparing their results with those of EOM-CCSD(SOC). According to our results, EOM(SOC)-CCSD is not necessarily more accurate than the perturbative methods and should not be used. Difference between rpSOC-EOM-CCSD and pSOC-EOM-CCSD is small and rpSOC-EOM-CCSD is an economical and reliable method in treating SOC effects particularly for excited states for systems containing elements up to the fifth row.
Disclosure statement
No potential conflict of interest was reported by the author(s).