Valence state ionization potential VSIP

The other energy matrix elements, Hf and H, are kept as in the conventional EHMO methodology, that is, the opposite value of the valence-state ionization potential (VSIP). 

A related topic is the computation of valence-shell ionization potentials (VSIP). The calculation of vertical ionization potentials via Koopmans theorem28 leads in many cases to serious errors, and a version of the ASCF method has been used to compute VSIP for several small molecules, including CH4.29 All the valence hole states of the molecule were computed. Agreement with experiment was substantially better than in the calculations using Koopmans theorem. 

VSIP vuv v/v VVk valence-state ionization potential vacuum ultraviolet volume per volume Van Vleck... 

The values for diagonal elements are taken as the negative of the average ionization potential for an electron in the appropriate valence orbital. Of course in many-election atoms, the valence-shell ionization potential (VSIP) for the ground-state atomic term may not necessarily be the best choice for the atom in a molecule, so this term is best regarded as an adjustable parameter, although one with a clear, physical basis. VSlPs have been tabulated for most of the atoms in the periodic table [211,212]. 

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