Coupled-cluster response theory for near-edge x-ray-absorption fine structure of atoms and molecules

Based on an asymmetric Lanczos-chain subspace algorithm, damped coupled cluster linear response functionshave been implemented for the hierarchy of coupled cluster (CC) models including CC with single excitations (CCS), CC2, CC with single and double excitations (CCSD), and CCSD with noniterative triple corrected excitation energies CCSDR(3). This work is a first step toward the extension of these theoretical electronicstructure methods of well-established high accuracy in UV-vis absorption spectroscopies to applicationsconcerned with x-ray radiation. From the imaginary part of the linear response function, the near K-edge x-ray absorption spectra of neon, water, and carbon monoxide are determined and compared with experiment.Results at the CCSD level show relative peak intensities in good agreement with experiment with discrepanciesin transition energies due to incomplete treatment of electronic relaxation and correlation that amount to 1–2 eV.With inclusion of triple excitations, errors in energetics are less than 0.9 eV and thereby capturing 90%, 95%,and 98% of the relaxation-correlation energies for C, O, and Ne, respectively.