Excitation energies in density functional theory: An evaluation and a diagnostic test

Peach, MJG; Benfield, P; Helgaker, T; Tozer, DJ

Abstract

Electronic excitation energies are determined using the CAM-B3LYP Coulomb-attenuated functional [ T. Yanai et al. Chem. Phys. Lett. 393, 51 (2004) ], together with a standard generalized gradient approximation (GGA) and hybrid functional. The degree of spatial overlap between the occupied and virtual orbitals involved in an excitation is measured using a quantity Λ, and the extent to which excitation energy errors correlate with Λ is quantified. For a set of 59 excitations of local, Rydberg, and intramolecular charge-transfer character in 18 theoretically challenging main-group molecules, CAM-B3LYP provides by far the best overall performance; no correlation is observed between excitation energy errors and Λ, reflecting the good quality, balanced description of all three categories of excitation. By contrast, a clear correlation is observed for the GGA and, to a lesser extent, the hybrid functional, allowing a simple diagnostic test to be proposed for judging the reliability of a general excitation from these functionals—when Λ falls below a prescribed threshold, excitations are likely to be in very significant error. The study highlights the ambiguous nature of the term “charge transfer,” providing insight into the observation that while many charge-transfer excitations are poorly described by GGA and hybrid functionals, others are accurately reproduced.

Citation

Peach, M., Benfield, P., Helgaker, T., & Tozer, D. (2008). Excitation energies in density functional theory: An evaluation and a diagnostic test. The Journal of Chemical Physics, 128(4), Article 044118. https://doi.org/10.1063/1.2831900

• Journal Article Type: Article
• Publication Date: Jan 1, 2008
• Deposit Date: Feb 6, 2012
• Publicly Available Date: Nov 2, 2012
• Journal: Journal of Chemical Physics
• Print ISSN: 0021-9606
• Electronic ISSN: 1089-7690
• Publisher: American Institute of Physics
• Peer Reviewed: Peer Reviewed
• Volume: 128
• Issue: 4
• Article Number: 044118
• DOI: https://doi.org/10.1063/1.2831900
• Keywords: Charge exchange, Density functional theory, Organic compounds, Rydberg states.
• Public URL: https://durham-repository.worktribe.com/output/1506680