MO-LCAO approach and Ab initio computations

The physical reliability and foundations of the Hund-Hückel MO-LCAO model, in which a molecular orbital (MO) is represented as a linear combination of atomic orbitals (LCAO), are examined in light of ab initio self-consistent field (SCF) computations with bases of various sizes. It is shown that appropriate linear transformations of the basis make evident a substructure of the relevant ab initio matrices and vectors that justifies the physical assumptions on which the MO-LCAO model rests. The main conceptual aspects, viz. hydrogen-like atomic orbitals, hybrids, localized orbitals, give a coherent picture, and the numerical data show that the contributions to occupied orbital energies ignored in the MO-LCAO model are actually small enough to justify their neglect in chemical structure studies. The numerical parameter values can therefore be read quantitatively in the ab initio results and the importance of the approximations made estimated. These conclusions have been drawn from studies on a variety of molecules with first- and second-row atoms. The well-established transferability of group properties in molecules suggests that they are generally valid. The claim that hybrids are method-independent, and therefore features of chemical reality at the orbital level, is discussed due to its relevance for comparisons of the stability of chemical bonds based on overlap. Of course, further work on questions such as electronegativities and promotion is needed. © 2006 Wiley Periodicals, Inc.