Electronic excited states in extreme limits via ensemble density functionals

Abstract

Ensemble density functional theory (EDFT) is a promising cost effective approach to address low-lying excitations in many-electron systems. Yet the corresponding functionals depend on the structure of the excitation in a challenging fashion; a fact which hinders a general and agile exploitation of the methodology. Here, we demonstrate that such a dependence become trivial as electrons become strictly correlated, because EDFT functionals become equivalent to pure ground state DFT functionals. This finding forms a most remarkable exact condition to deal with excitations in the low-density limit of all systems. An immediate consequence of which, excitations in Wigner systems can be dealt with non-empirically and universally.