An analysis of configuration-interaction model spaces for the study of the core photoelectron spectra of small molecules

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Abstract
Two-hole, one-particle (2h-1p), 3h-2p and 4h-3p configuration-interaction (CI) calculations of the shake-up in the core photoelectron spectra of H2O, HOF, F2, F2O, N2, CO and O3 have been compared with full configuration interaction (FCI) in a minimal basis set. Explicit inclusion of relaxation is found to be important even at the 3h-2p level, while no definite advantage of the delocalised description is observed beyond the 2h-1p level. The relaxed 3h-2p scheme is found to be adequate for the low-energy part of the spectra; higher excitations become important at higher energy. Both the important correlations and the molecular relaxation appear to be well described within the minimal basis. However, extension of the basis is essential for a quantitative description of the spectrum. The effect of an extended basis has been analysed for the ionisations of CO, using a projection scheme which allows a better definition of the empty orbitals, while keeping the conceptual simplicity of the valence space.