Building on the Strengths of a Double-Hybrid Density Functional for Excitation Energies and Inverted Singlet-Triplet Energy Gaps

It is demonstrated that a double hybrid density functional approximation (DH-DFA), ωB88PTPSS, that incorporates equipartition of density functional theory (DFT) and non-local correlation however with a meta-GGA correlation functional as well as with the range-separated exchange of ωB2PLYP provides accurate excitation energies for conventional systems as well as correct prescription of negative singlet-triplet gaps for non-conventional systems with inverted gaps, without any necessity for parametric scaling of the same-spin and opposite-spin non-local correlation energies. Examined over " safe" excitations of the QUESTDB set, ωB88PTPSS performs quite well for open-shell systems, correctly and fairly accurately (relative to EOM-CCSD reference) predicts negative gaps for 50 systems with inverted singlet-triplet gaps, is one of the leading performers for intramolecular charge-transfer excitations and achieves near-CC2 and ADC(2) quality for the Q1 and Q2 subsets. Subsequently, we tested ωB88PTPSS on two sets of real-life examples from recent computational chemistry literature; the low energy bands of chlorophyll a (Chl a) and a set of thermally activated delayed fluorescence (TADF) systems. For Chl a, ωB88PTPSS quantitatively and quantitatively achieves DLPNO-STEOM-CCSD-level performance and provides excellent agreement with experiment. For TADF systems, ωB88PTPSS agrees quite well with SCS-CC2 excitation energies as well as experimental values for the gaps between the S1 and T1 excited states.