Accurate yet feasible post-Hartree-Fock computation of magnetic interactions in large biradicals through a combined variational/perturbative approach: setup and validation

We present a scheme for the calculation of the spin-spin coupling term J in diradicals which is quantitatively accurate and computationally cheap. The method exploits the use of modified virtual orbitals and perturbation theory, incorporated in a multireference configuration interaction approach. The results obtained for model diradical species which exhibit ferromagnetic and antiferromagnetic coupling are fully satisfactory and very promising for future applications of the method to larger molecular systems of technological interest in magnetic-based devices.