title: Excitation Energies in Time-Dependent (Current-) Density-Functional Theory: A Simple Perspective.
abstract: This paper gives a simple and pedagogical explanation, using density matrices of two-level systems, how to calculate excitation energies with time-dependent density-functional theory. The well-known single-pole approximation for excitation energies is derived here in an alternative way and extended to time-dependent current-density-functional theory.
authors.affiliation: Department of Physics and Astronomy, University of Missouri, Columbia, Missouri 65211.
authors.forename: C A
authors.initials: CA
authors.lastname: Ullrich