Multicenter distribution of molecular integrals and energy components

A central problem in both semiempirical and ab initio molecular electronic structure theory is treatment of the three- and four-center interactions. A systematic evaluation of multicenter contributions and a search for invariants has been carried out by computing frequency distribution functions for one-, two- versus three-, four-center two-electron integrals and by separating one-, two-center from three-, four-center terms in the one-electron energies of molecular-orbital theory. Data have been obtained for a considerable number of polyatomic molecules and representative results are presented for NH−2, NH3, CH4, H3O+, C2N2, C3H+3, N4, and C6H6.