ON CHEMICAL BONDING BETWEEN MOLECULAR FRAGMENTS - CLOSED-SHELL FRAGMENTS

The chemical bonding between two closed-shell molecular fragments has been analyzed in terms of comparison of the antisymmetrized product of wave functions of the fragments M(PHI)BAR with the wave function of a molecule composed of these fragments M(PHI). As a measure of the frag ments' electronic structure variation upon molecule formation we took the cosine of the angles between M(PHI) and M(PHI)BAR in Hilbert space , as well as between the corresponding first-order density matrices M( rho) and M(rho)BAR. As an example, compounds of the type BH3L (L = CO, NH3, PH3, H2O, PF3) have been considered. It is stressed that a neces sary condition of chemical bonding between closed-shell fragments is t he contribution of vacant orbitals of fragments into the bond