Abstract
The relaxed-core Hartree-Fock (RCHF) approximation is applied to discrete K-shell (or deep inner shell) excitations in molecules. As an example, the O 1s excitations in H2O are considered. A recently proposed method is used to evaluate the N-electron transition moments and to deal with the concomitant non-orthogonality problem arising from the “frozen” and “relaxed” orbital representations of the intiial and final states, respectively. The comparison of the RCHF results for H2O with those of a largescale polarization propagator treatment shows a very satisfactory agreement both for term values and oscillator strengths. By contrast, only poor results are obtained in the frozen-core Hartree-Fock (FCHF) scheme neglecting the relaxation of the valence electrons in the final state. Due to its simplicity the RCHF approximation allows one to consider large molecules and to employ rather extended SCF basis sets. Thus, this method may be particularly useful to determine quantum defect parameters for core-excited Rydberg states in polyatomic molecules.