State-Specific Density Functionals for Excited States via a Density-Driven Correlation Model

We present a first principles strategy for developing approximations for excited states through ensemble density functionals. Central to our result is the recognition that density-driven correlations (ddc's) can be vitally important to address excited states individually through ensembles, yet standard density-functional approximations based on ground state physics miss ddc's altogether. To model the ddc, we exploit the recently understood low-density limit of electrons in excited states. The theory developments are then combined to produce a proof-of-concept excited state approximation that resolves urgent paradigmatic failures (double excitations, charge transfer excitations, piecewise linearity) of existing state-of-art density-functional approaches, directly from differences in self-consistent field calculations; i.e., ΔSCF. In light of its observed impressive performance, we conclude that the approach represents a major step toward unified and accurate modeling of neutral and charged excitations.