Merging multiconfigurational wavefunctions and correlation functionals to predict magnetic coupling constants.

We study the performance of different approaches that combine multiconfigurational wavefunctions with correlation functionals for the calculation of magnetic coupling constants of several materials and molecules. The systems under study include four antiferromagnetic materials: NiO, KNiF(3), K(2)NiF(4) and La(2)CuO(4); two biradicals: alpha-4-Dehydrotoluene and 1,1',5,5'-Tetramethyl-6,6'-dioxo-3,3'-biverdazyl; two molecular complexes: Cu(2)Cl(6) and Copper(II) acetate monohidrate; and the prototypical H-He-H system. On average, the best results are obtained with a recently proposed method [Phys. Rev. A 75, 012503 (2007)] that estimates the correlation energy of density functionals from a pair of alternative spin densities built from the natural orbitals and occupation numbers of the multiconfigurational wavefunction.