Orbital-optimized multiconfiguration VB methods

The peptide and thio-peptide bonds have some specific properties like coplanarity, substantial rotational barrier and kinetic stability towards nucleophilic attack or hydrolysis. All these properties are easily rationalized by the 

As in the preceding case, the simple resonance picture was critic-on the basis of electron population analyses using the AIM meth-od, to the extent that (thio)amides were proposed to be viewed as special 

Clearly, therefore, VBSCF constitutes a handy tool for studies of the role of electronic delocalization, in molecules that possess more than one Lewis 

VBCI A Post VBSCF Method that Involves Dynamic Correlation 

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. 

---