Journal
JOURNAL OF CHEMICAL PHYSICS
Volume 131, Issue 23, Pages -Publisher
AMER INST PHYSICS
DOI: 10.1063/1.3269030
Keywords
ab initio calculations; density functional theory; orbital calculations
Funding
- European Union
- Unam-Regpot [203953]
- TUBITAK [109T426]
- Bilkent University
- Israel Science Foundation [962/06]
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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.
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