Basic exchange model: Comparison of Anderson and valence bond configuration interaction approaches and an alternative exchange expression

The application of the Anderson [Phys. Rev. 115, 2 (1959)] and valence bond configuration interaction (VBCI) methods on a basic exchange model is investigated to assess their ability to calculate the exchange energy under a variety of physical situations, represented by the model parameters. The model comprises two metal orbitals and a central ligand orbital. We show that the validity of the Anderson method is not restricted to cases in which U