Canonical-ensemble SA-CASSCF strategy for problems with more diabatic than adiabatic states: Charge-bond resonance in monomethine cyanines

This paper reviews basic results from a theory of the classical probabilities in state-averaged complete active space self-consistent field wave functions. It is proposed that a canonical (Boltzmann) weighting scheme makes the self-consistency condition invariant with respect to configurational transformations that act locally on the support of the equilibrated SA-CASSCF ensemble. This is illustrated with a complete active space valence-bond (CASVB) analysis of low-energy electronic states in a monomethine cyanine dye.