Relativistic equation-of-motion coupled-cluster theory (EOM-CC)

Abstract

Equation-of-motion coupled-cluster (EOM-CC) theory can be employed to calculate excitation energies (EE), ionization potentials(IP), as well as electron affinities (EA). The EOM-CC approach at the CC singles and doubles level (CCSD) is able to provide EEs, IPs, and EAs with an error of about 0.1-0.3 eV for single-excitation states or Koopmans states from a reference with a dominant single-reference character. Scalar-relativistic effects can be incorporated straightforwardly in EOM-CC calculations when untransformed two-electron interactions are adopted. On the other hand, time-reversal symmetry and spatial symmetry of double point groups need to be exploited to achieve an efficient implementation when spin-orbit coupling (SOC) is present. Furthermore, including SOC in post-self-consistent field (SCF) treatment could result in a further reduction in computational effort particularly for molecules with low symmetry. Due to effective treatment of orbital relaxation effects by single excitations in the cluster operator, this approach can afford accurate description on SOC effects. It is nontrivial to impose time-reversal symmetry for open-shell reference and broken time-reversal symmetry could result in spurious-level splitting. Openshell system with one-unpaired electron can be calculated based on EOM-CC for IPs or EAs from a closed-shell reference. In addition, Kramer’s degeneracy has to be taken into consideration when calculating properties of systems with an odd number of electrons using EOM-CC for IPs or EAs from a closedshell reference. EEs, IPs, and EAs for systems containing heavy elements can be obtained reliably based on EOM-CCSD approaches, and SOC splitting is calculated with reasonable accuracy even for double-excitation states.