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.