A tuned double hybrid range-separated functional: Accurate reproduction of inverted singlet-triplet gap

Conventional density functional theory (DFT) is unable to predict inverted singlet-triplet gaps, largely because the underlying Kohn-Sham framework does not capture correlation effects arising from double excitations. This limitation has hindered the accurate description of systems where singlet-triplet inversion plays a key role, for example, in thermally activated delayed fluorescence emitters. To address this issue, we develop a modified long-range corrected functional, LC-BLYP(D), that augments the ground-state description with MP2 correlation and incorporates CIS(D) correlation for the excited state. By explicitly accounting for correlation effects that are absent in conventional functionals, LC-BLYP(D) provides a more balanced treatment of ground- and excited-state energetics. In addition, we introduce a single-step tuning protocol for the range-separation parameter, designed to optimize the performance of LC-BLYP(D) without the need for iterative procedures. When applied in combination, the tuned LC-BLYP(D) functional reproduces inverted singlet-triplet gaps with a mean absolute error of 0.022 eV, representing a substantial improvement over standard DFT approaches. These results demonstrate that the incorporation of correlated wavefunction-based corrections into a range-separated framework not only overcomes a fundamental shortcoming of DFT but also offers a practical and accurate tool for investigating excited-state properties in complex molecular systems.