On the performance of the hybrid TPSS meta-GGA functional to study the singlet open-shell structures: A combined theoretical and experimental investigations of the Ni2O2 molecule

Compared with the experimental data, we showed that DFT (and TDDFT) calculations provide a reliable description of the photophysical process, provided the following requirements. We have to: (a) use a hybrid meta-GGA functional within the unrestricted DFT formalism (UDFT) for all spin multiplets, (b) determine a stable electronic solution for each multiplet (particularly for the singlet state), in order to give a qualitatively correct description of a multireference system, (c) determine the exact exchange contribution in the hybrid exchange-correlation functional, as the most stable state of each isomer is the singlet. For the Ni2O2 molecules, we found that the last requirement could be satisfied for the Hartree-Fock exchange contribution ranging from 2% to 8%, notably smaller than the value (10%) suggested for the TPPSh hybrid meta-GGA functional. Consequently, it is shown that the UDFT description covers static correlation effects needed for the correct treatment of the Ni2O2 isomers.