Orthogonal Valence Bond Hamiltonians incorporating dynamical correlation effects

The Complete Active Space Self-Consistent Field calculations offer a definition of a set of molecularly optimized valence mono-electronic functions and a basis of Orthogonal Valence Bond (OVB) configurations. However this variational description misses important dynamical correlation effects, affecting both the energy and the weights of the various OVB components. The dynamical polarization effects induce for instance an increase of the weight of the ionic VB components. The mapping of these dynamical correlation effects into an effective OVB Hamiltonian is not straightforward, but would be useful for the production of reliable model Hamiltonians, making possible the treatment of large systems. The present work takes benefit of a solution proposed to the multi-parentage problem in Multi-Reference Coupled Cluster methods to define in a rational way dressed OVB Hamiltonians. A test study on the F2molecule shows that the dynamical correlation not only diminishes the energy difference between the neutral and ionic configurations, but also the interaction between them.