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.