Ab Initio Calculation of One‐Center Integrals of Semiempirical Theories of Valence

In order to highlight some of the apparent contradictions between semiempirical and ab initio theories of valence, a brief review of some facets of the two approaches is presented. The recently developed effective valence shell Hamiltonian, , is then briefly introduced to bridge the gap. Results of purely ab initio H v calculations for atoms and simple molecules are mentioned. The ab initio one‐center integrals are quantitatively much different from those of semiempirical theories. However, the exact contains extra terms, three‐ (or higher) electron interactions, which are totally absent in semiempirical theories. This paper shows that the nonclassical, three‐electron terms are incorporated into semiempirical one‐ and two‐electron integrals, and a parametrization scheme illustrating this is presented. As an example, the carbon system is chosen and the one‐center carbon semiempirical integrals are calculated from the results of ab initio calculations for C and CH. This enables discussions of questions of the degree of transferability of semiempirical one‐center integrals. The one‐center one‐ and two‐electron integrals obtained from ab initio calculations by including the effects of three‐electron terms are found to be close to the traditional semiempirical values.