Relativistic EOM-CCSD for Core-Excited and Core-Ionized State Energies Based on the Four-Component Dirac–Coulomb(−Gaunt) Hamiltonian

We report an implementation of the core–valence separation approach to the four-component relativistic Hamiltonian-based equation-of-motion coupled-cluster with singles and doubles theory (CVS-EOM-CCSD) for the calculation of relativistic core-ionization potentials and core-excitation energies. With this implementation, which is capable of exploiting double group symmetry, we investigate the effects of the different CVS-EOM-CCSD variants and the use of different Hamiltonians based on the exact two-component (X2C) framework on the energies of different core-ionized and -excited states in halogen- (CH3I, HX, and X–, X = Cl–At) and xenon-containing (Xe, XeF2) species. Our results show that the X2C molecular mean-field approach [ Sikkema, J. ; J. Chem. Phys. 2009, 131, 124116 ], based on four-component Dirac–Coulomb mean-field calculations (2DC M ), is capable of providing core excitations and ionization energies that are nearly indistinguishable from the reference four-component energies for up to and including fifth-row elements. We observe that two-electron integrals over the small-component basis sets lead to non-negligible contributions to core binding energies for the K and L edges for atoms such as iodine or astatine and that the approach based on Dirac–Coulomb–Gaunt mean-field calculations (2DCG M ) are significantly more accurate than X2C calculations for which screened two-electron spin–orbit interactions are included via atomic mean-field integrals.