DESCRIPTION OF CORE-EXCITATION SPECTRA BY THE OPEN-SHELL ELECTRON-ATTACHMENT EQUATION-OF-MOTION COUPLED-CLUSTER METHOD

The theoretical description of core-excitation spectra presents a difficult problem due to the large excitation energies involved, and the extensive relaxation effects that occur upon promotion of a core electron to a valence or Rydberg level. For this reason we follow a two-step procedure to evaluate core-excitation energies. We start from a coupled cluster singles-doubles (CCSD) description of the core ion to include the large relaxation effects, followed by adding an extra electron to the core-ionized state to obtain the various core-excited states of the neutral by using the open-shell electron attachment equation-of-motion coupled cluster method (EA-EOMCC). An important feature of the approach is that the term values, the core-excitation energies relative to the relevant core-ionization potential, are calculated directly and this allows us to achieve high accuracy. This work describes the extension of the EA-EOMCC method to open-shell reference states and we make applications to a number of molecular systems. The assignment of recently obtained high-resolution core-excitation spectra for acetylene and ethylene is discussed, and we compare our open-shell EA-EOMCC results to results obtained from closed-shell EA-EOMCC calculations based on the equivalent core ion corresponding to the core-excited molecular system. Special attention is paid to the singlet-triplet splitting for core-excited states, and we address the multireference character of core-ionized and core-excited states for molecules that contain symmetry-equivalent heavy nuclei, which relates to a persistent controversy in the literature concerning localized versus delocalized core holes. © 1995 American Institute of Physics.