Natural bond orbitals in multiconfigurational expansions: Local treatment of electron correlation in molecules

We describe a localized treatment of electron correlation in terms of complete active-space wave functions derived from natural bond orbitals, closely related to the localized transferable units of molecular structure. We find that in many cases the procedure leads to solutions which retain the essentially localized character of their ‘‘parent’’ orbitals. Such localized solutions lead to an extremely selective treatment of electron correlation in molecules, allowing one to readily extend the concepts of electron correlation in diatomic molecules (‘‘left-right,’’ ‘‘in-out,’’ ‘‘angular,’’ etc.) to the general polyatomic case using a familiar chemical language. We illustrate the procedure with applications to various diatomic (Li2, CO), triatomic (H2O, symmetric and unsymmetric dissociation), and polyatomic (formaldehyde ground and excited states; malonaldehyde) species, in order to demonstrate the important advantages of this procedure with respect to numerical behavior (improved convergence, reduced dimensionality, easy implementation with standard program packages) and conceptual utility (selectivity, transferability, description in a language common to all chemists).