Assessment of the Piris natural orbital functionals on transition metal dihydrides

Transition metal dihydrides can be seen as a starting point for the study of the interaction between H₂ and d-metal ligands that are widely recognized as challenging molecules for electronic structure methods due to their possible multireference character and electronic correlation effects. The performance of different proposed Piris natural orbital functionals (PNOFs), such as PNOF5, PNOF7, and the global functional (GNOF), in predicting the formation of 3d-transition metal dihydrides was evaluated. A comparison between the results of the PNOFs and several state-of-the-art techniques has been carried out. It was found that all PNOF methods are consistent when static correlation effects are negligible. In particular, PNOF7 demonstrated the most accurate ratio for predicting the formation of the dihydrides and energy profiles, according to multireference methods. On the other hand, although the GNOF method successfully predicts the equilibrium geometries, it overstabilizes systems with high static correlation, such as low multiplicity dihydrides of intermediate 3d-series transition metals. As a whole, they indicate activation of the H₂ bond for all metals and also the formation of dihydrides, except for Co, Cu and Zn.