EQUIVALENT-CORE CALCULATION OF CORE-LEVEL RELAXATION ENERGIES IN PHOTOELECTRON-SPECTROSCOPY - A MOLECULAR-ORBITAL APPROACH

The equivalent-core approximation is implemented in a novel way so as to calculate core-level relaxation energies in photoelectron spectrosc opy. The method is based on self-consistent field (SCF) Hartree-Fock m olecular-orbital calculations via linear combinations of atomic orbita ls, and involves evaluating the difference of sums of two-electron Cou lomb and exchange integrals, for all electrons in an atom and in its e quivalent-core ion. By thus avoiding SCF calculations with a core hole present (the true final state of photoemission), this procedure is sh own to significantly save computing time in comparison with an exact S CF direct-hole calculation. Application of the method in single atoms and selected molecules shows about a 10% difference with respect to di rect-hole calculation results. The approximation introduces about 1-6 eV errors compared to the experimental results of gas phase molecules. This method thus should be a generally useful procedure for estimatin g relaxation energies in core spectra. (C) 1998 American Institute of Physics. [S0021-9606(98)30540-1].