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

TL;DR

This paper introduces a canonical-ensemble SA-CASSCF approach that improves the treatment of systems with more diabatic than adiabatic states, demonstrated through charge-bond resonance in monomethine cyanines.

Contribution

It proposes a Boltzmann weighting scheme for SA-CASSCF that maintains invariance under local configurational transformations, enhancing the analysis of complex electronic states.

Findings

Canonical-ensemble SA-CASSCF is effective for systems with many diabatic states.

The method provides a consistent framework for state-averaged wave functions.

Application to monomethine cyanines reveals detailed charge-bond resonance insights.

Abstract

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.