AbstractIn order to highlight some of the apparent contradictions between semiempirical andab initiotheories 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 purelyab initioHvcalculations for atoms and simple molecules are mentioned. Theab initioone‐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 ofab initiocalculations 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 fromab initiocalculations by including the effects of three‐electron terms are found to be close to the traditional