The role of hybridization in perturbative bond theories: The existence of exact strictly localized orbitals in small molecules

Abstract

The significance of hybrid atomic orbitals is discussed in the context of perturbation theories of the chemical bond. It is emphasized that the correct choice of hybrids is essential in obtaining sufficiently accurate zeroth order (strictly localized) solution, representing a “Pauling-point” in chemical-bond theories. It is proved that for some small molecules, e.g. for first-row hydrides in small basis sets, there may exist a set of non-orthogonal exactly localized (two-centre) occupied molecular orbitals which span the same subspace and have the same energy as do the canonical MOs, but have no tails on other atoms. Although this statement cannot be generalized to larger molecules or larger basis sets, it still suggests that the apparent interbond delocalization (tails) predicted by various perturbative schemes does not necessarily correspond to physical interbond interaction, but, to a large extent, it may be a consequence of using poor atomic hybrids. As examples, non-orthogonal strictly localized MOs are reported for CH 4 and NH 3 in the STO-3G basis set.