Excitation Energies of Molecules from Ensemble Density Functional Theory

Abstract

Abstract Ensemble methods for excited states are based on the ensemble variation principle and in their simplest formulations can be either based on the wavefunction or the electron density. The latter group shares the favorable scaling of ground state density functional theory (DFT) and as such can be considered a computationally inexpensive alternative to time-dependent (TD)-DFT in cases where TD-DFT is not sufficiently accurate. The failures of TD-DFT most prominently include the poor description of conical intersections and excitations of multiple character, i.e., when multiconfigurational effects play a significant role. To deal with such issues, quite recently a number of multiconfiguration ensemble methods have been designed that combine a wavefunction-based formulation with ensemble density functional theory. This chapter discusses the merits and shortcomings of such approaches. It also attempts to elucidate some of the essential problems associated with the ensemble DFT methods and their variants to the computational chemistry community.