Toward a variational treatment of the magnetic coupling between centers with elevated spin moments

A multireference configuration interaction scheme is presented to calculate electronic structure parameters for systems with an elevated number of unpaired electrons. The paper concentrates on the magnetic coupling but the method can also be applied to other parameters. The reference wave function contains not only the usual configurations contained in the Anderson model but is extended with ligand-to-metal charge transfer configurations. Subsequently, a small subset of the complete difference dedicated configuration interaction space is included in the calculation. Different strategies to introduce the charge transfer configurations in the reference wave function are compared. Projected model ligand vectors ensure the optimal inclusion of the charge transfer effects and good agreement with more extensive calculations is obtained at a more reduced computational cost.