Application of a “charge-averaged” second order multireference perturbation theory strategy to the study of a model Mixed-Valence compound

A new proposed second order Multireference Perturbation Theory (MRPT) approach (termed NEVPT2(av)), based upon the use of state-averaged canonical molecular orbitals instead of the state-specific canonical ones, is applied to the study of a model Mixed-Valence compound. Such strategy has already been shown, on the basis of some preliminary calculations reported in a previous paper, to be able to overcome the inadequacy of a standard second order MRPT treatment for such MV systems. In order to test the validity and the firmness of the NEVPT2(av) methodology, an investigation is carried out, using basis sets of various size and two different active spaces. The reliability of the method is fully confirmed by the good agreement achieved with the results of highly-correlated Multireference Configuration Interaction (MRCI) computations recently published.