A valence-bond-based complete-active-space self-consistent-field method for the evaluation of bonding in organic molecules

We present the complete-active-space self-consistent-field (CASSCF) implementation of a valence-bond (VB) based method for the analysis of bonding in organic molecules. The method uses the spin-exchange density matrix P with a localized orbital basis, where the determinants of the CASSCF wavefunction become VB-like determinants with different spin coupling patterns. The index P-ij evaluates the contributions of the determinants to the CASSCF wavefunction and is used to generate resonance formulas. We use the bonding contributions in the original VB formulation of the method (alphabeta terms). The method is applied in studies of excited-state reactivity, as shown here for indole. Its first excited state is covalent and is characterized by a decrease in the bond orders in the benzene moiety, similar to the B-2u excited state of benzene. In contrast, the ionic excited state has an inversion in the bonds of the pyrrole moiety induced by charge transfer to the benzene ring.