Lewis bond order for some controversial canonical molecular orbitals

The Lewis structure, consisting of core, lone-pair and bonding-type localized molecular orbitals, can give an unambiguous bonding picture and an intuitive bond order. Therefore a new scale of bond order, the Lewis bond order, can be assigned to localized molecular orbitals in terms of intuitive Lewis structures. The Lewis bond orders of canonical molecular orbitals can be obtained by expanding them into localized molecular orbitals with the assigned Lewis bond orders. Test calculations have been performed for the molecules N-2, O-2, F-2, P-2, S-2, CO, CS, NO and SO, whose Hartree-Fock wavefunctions can be well localized. The total Lewis bond orders of these molecules turn out to be exactly the bond orders revealed by the Lewis structures. For canonical molecular orbitals with an ambiguous bonding nature, e.g. 2 sigma(g), 2 sigma(u) and 3 sigma(g) of the molecule N-2, their Lewis bond orders match quite well with the photoelectron spectroscopy results. (C) 1997 Elsevier Science B.V.