Single-electron excitations from thermally-assisted-occupation reference

The linear response time-dependent density functional theory (LR-TDDFT) has been broadly used to investigate excited-state properties of various molecular systems. However, current LR-TDDFT methods heavily rely upon outcomes from ground-state DFT calculations. For systems with small HOMO-LUMO gaps, single-determinant ground-state DFT may be prone to non-dynamical correlation, and hence, LR-TDDFT results can be inaccurate. Traditionally, nearly degenerate orbitals require proper treatment of non-dynamical correlation, which usually involves active-space treatments. Recently, the thermally-assisted-occupation (TAO) DFT scheme was proposed, which explicitly incorporates the non-dynamical correlation effect in the ground-state simulation, but retains the low computational complexity of conventional DFT. The aim of this work is to combine ground-state TAO-DFT with LR-TDDFT framework to study excited-state phenomena. The preliminary simulations successfully capture the dissociation feature of the first triplet excited state of hydrogen molecule, which is not the case for conventional TDDFT.