Dissociation energies field wave functions

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). 

Clearly, the errors introduced by the approximations should be as small as possible. Nevertheless, in most molecular calculations, the absolute errors will still be large compared with energies of chemical interest. Such calculations may nevertheless be meaningful if the errors can be controlled or cancelled in some manner. Thus, our approximations should be designed so as to yield solutions of the same quality for the systems and physical situations to be compared. For example, in dissociation processes, the approximations should give results of similar quality for the reactants and the products. To describe correctly the response of the wave function to external perturbations such as electric and magnetic fields, the approximations should give a balanced treatment of the electronic system in the field and out of the field. 

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