Post-B3LYP Functionals Do Not Improve the Description of Magnetic Coupling in Cu(II) Dinuclear Complexes

The accuracy of post-B3LYP functionals is analyzed using an open shell database of Cu(II) dinuclear complexes with well defined experimental values of the magnetic coupling constants. This database provide a sound open shell training set to be used to improve the fitting schemes in defining new functionals or when reparametrizing the existing ones. For a large set of representative hybrid functionals, it is shown that the overall description of moderate to strong antiferromagnetic interactions is significantly more accurate than the description of ferromagnetic or weakly antiferromagnetic interactions. In the case of global hybrids, the most reliable ones have 25-40% Fock exchange with SOGGA and PBE0 being the most reliable and M06 the exception. For range corrected hybrids, the long range corrected CAM-B3LYP and B97XD provide acceptable results and M11 is comparable but more erratic. It is concluded that he reliability of the calculated values is system and range dependent and this fact introduces a serious warning on the blind use of a single functional to predict magnetic coupling constants. Hence, in order to extract acceptable magnetostructural correlations, a “standardization” of the method to be used is advised to choose the optimal functional.