title: Koopmans' springs to life.
abstract: The meaning of orbital energies (OOEs) in Kohn-Sham (KS) density functional theory (DFT) is subject to a longstanding controversy. In local, semilocal, and hybrid density functionals (DFs) a Koopmans' approach, where OOEs approximate negative ionization potentials (IPs), is unreliable. We discuss a methodology based on the Baer-Neuhauser-Livshits range-separated hybrid DFs for which Koopmans' approach "springs to life." The OOEs are remarkably close to the negative IPs with typical deviances of +/-0.3 eV down to IPs of 30 eV, as demonstrated on several molecules. An essential component is the ab initio motivated range-parameter tuning procedure, forcing the highest OOE to be exactly equal to the negative first IP. We develop a theory for the curvature of the energy as a function of fractional occupation numbers to explain some of the results.
authors.affiliation: Department of Chemistry, Bilkent University, 06800 Bilkent, Ankara, Turkey. salzner@fen.bilkent.edu.tr
authors.forename: Ulrike
authors.initials: U
authors.lastname: Salzner
authors.forename: Roi
authors.initials: R
authors.lastname: Baer