Configuration Interaction Involving Ionic Terms

The VBCI method, recently developed by Wu et al. is a post-VBSCF calculation that uses configuration interaction to supplement the VBSCF energy with dynamic correlation. At the same time, the method preserves the interpretability of the final wave function in terms of a minimal number of VB structures, each having a clear chemical meaning. The VB structures that are used in the VBSCF calculations are referred to as fundamental structures, denoted as T , and the orbitals that appear in the VBSCF calculation are referred to as occupied orbitals. Depending on the problem at hand, the VBSCF calculation may use semidelocalized CF orbitals, or orbitals that are each localized on a single atom or fragment in the latter case the fundamental structures will explicitly involve the covalent and ionic components of the bonds. 

Porter and Karplus [19] constructed a LEPS potential for H + H2 including overlap and three-center terms in order to evaluate the energies of nonlinear configurations more realistically. Kuntz et al. [313] employed a modified LEPS function in a detailed investigation of metathetical reactions involving three atoms. Three adjustable parameters were included, instead of just S2. This provided a more flexible potential, and it was possible to vary the nature of the potential surface quite considerably. Other potentials based on pairwise interactions have been used for calculations where AB is ionic [72-74, 306],... 

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