The role of the magnetic orbitals in the calculation of the magnetic coupling constants from multireference perturbation theory methods

摘要

The use of multireference perturbation theory (MRPT) for the calculation of the magnetic couplingin binuclear complexes has shown to give poor results if applied on a minimal active space completeactive space self-consistent field (CASSCF) wavefunction. In this work, we identify the origin ofthis problem in the starting CASSCF orbitals, which are exceedingly localized on the metal atoms.Focusing on the case of antiferromagnetic systems, it is shown that the form of the active orbitalshas a dramatic effect on the relative description of the neutral and ionic structures. Finally, a simpleand computational inexpensive strategy is proposed for the calculation of a set of magnetic orbitalsdescribing in a more balanced way the neutral and ionic structures. The use of these orbitals, insteadthe CASSCF ones, in minimal active space MRPT2 calculations leads to a marked improvementof the J values, which become in reasonable agreement with those obtained with the expensivehigh level difference dedicated configuration interaction approach and with the experimental values