A Fermi smearing variant of the Tamm-Dancoff approximation for nonadiabatic dynamics involving S

The main shortcoming of time-dependent density functional theory (TDDFT) regarding its use for nonadiabatic molecular dynamics (NAMD) is its incapability to describe conical intersections involving the ground state. To overcome this problem, we combine Fermi smearing (FS) DFT with a fractional-occupation variant of the Tamm-Dancoff approximation (TDA) of TDDFT in the generalized gradient approximation. The resulting method (which we denote as FS-TDA) gives access to ground- and excited-state energies, gradients, and nonadiabatic coupling vectors, which are physically correct even in the vicinity of S