Complete active space valence bonds CASVB

H. Nakano, K. Sorakubo, K. Nakayama, K. Hirao, in Valence Bond Theory, D. L. Cooper, Ed. Elsevier, Amsterdam, The Netherlands, 2002, pp. 55-77. Complete Active Space Valence Bond (CASVB) Method and its Application in Chemical Reactions. 

Complete active space valence bond (CASVB) method and its application to chemical reactions... 

The complete active space valence bond (CASVB) method is an approach for interpreting complete active space self-consistent field (CASSCF) wave functions by means of valence bond resonance structures built on atom-like localized orbitals. The transformation from CASSCF to CASVB wave functions does not change the variational space, and thus it is done without loss of information on the total energy and wave function. In the present article, some applications of the CASVB method to chemical reactions are reviewed following a brief introduction to this method unimolecular dissociation reaction of formaldehyde, H2CO — H2+CO, and hydrogen exchange reactions, H2+X — H+HX (X=F, Cl, Br, and I). 

The complete active space valence bond (CASVB) method [1,2] is a solution to this problem. Classical valence bond (VB) theory is very successful in providing a qualitative explanation for many aspects. Chemists are familiar with the localized molecular orbitals (LMO) and the classical VB resonance concepts. 

Valence-bond methods have increased its applicability recently. One example is the CASVB (complete active space valence bond) method. A CASVB wave function can be obtained simply by transforming a canonical CASSCF function and readily interpreted in terms of the well-known classical VB resonance structures. The total CASVB wave function is identical to the canonical CASSCF wave function. In other words, the MO description and the VB description are equivalent, at least at the level of CASSCF. The CASVB method provides an alternative tool for describing the correlated wave functions. 

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