A COMPLETE ACTIVE SPACE VALENCE-BOND (CASVB) METHOD

A complete active space valence bond (CASVB) method is proposed which is particularly adapted to chemical interpretation. A CASVB wave funct ion can be obtained simply by transforming a canonical CASSCF function and readily interpreted in terms of the well known classical VB reson ance structures. The method is applied to the ground and excited state s of benzene, butadiene, and the ground state of methane. The CASVB af fords a clear view of the wave functions for the various states. The e lectronic excitation is represented in a VB picture as rearrangements of the spin couplings or as charge transfers which involve breaking co valent bonds and forming new ionic bonds. The former gives rise to cov alent excited states and the latter to ionic excited states. The physi cal reasons why it is so difficult to describe the ionic excited state s at the CASSCF level with a single active space and why the lowest 1 B-1(2)+ state in cis-butadiene is so stabilized compared to the corres ponding 1 B-1(u)+ state in the trans isomer are easily identified in v iew of a VB picture. The CASVB forms a useful bridge from molecular or bital theory to the familiar concepts of chemists. (C) 1996 American I nstitute of Physics.