Vertical Excitation Energies of Organic Molecules Calculated by the MC23 Hybrid Meta On-Top Functional as Compared to Other Multireference Methods.

The success of multiconfiguration pair-density functional theory depends on the quality of the on-top functional. We have tested the recently published MC23 hybrid meta on-top pair-density functionals, three other on-top functionals, CASPT2, and XMS-CASPT2 against the benchmark results of Loos et al. [J. Chem. Theory Comput. 2020, 16, 1711-1741] for 102 singlet-singlet and 65 singlet-triplet transition excitation energies of organic molecules with 4-6 nonhydrogenic atoms. We find that the tPBE, tPBE0, and MC23 on-top functionals all yield more accurate results on average than CASPT2 by factors of 1.3-1.7. The excitation energies are more accurate for singlet-triplet transitions than for singlet-singlet transitions, and they are more accurate with manually determined active spaces than with automatically generated ones.