A new and efficient equation-of-motion coupled-cluster framework for core-excited and core-ionized states
We present a fully analytical implementation of the core-valence separation (CVS) scheme for the equation-of-motion (EOM) coupled-cluster singles and doubles (CCSD) method for calculations of core-level states. In the spirit of the original CVS approximation proposed by Cederbaum, Domcke and Schirmer, pure valence excitations are excluded from the EOM target space and the frozen-core approximation is imposed on the reference-state amplitudes and multipliers. This yields an efficient, robust, and accurate EOM-CCSD framework for calculations of excitation and ionization energies as well as state and transition properties (e.g., spectral intensities, natural transition and Dyson orbitals). The accuracy of the new scheme is improved relative to the results obtained applying the CVS only during the solution of the EOM eigenvalue equations. The errors in absolute excitation/ionization energies relative to the experimental reference data are of the order of 0.2-3.0 eV, depending on the K-edge considered and on the basis set used, and the shifts are systematic for each edge.