title: A practicable real-space measure and visualization of static electron-correlation effects.
abstract: The inclusion of dynamical and static electron correlation (SEC) is mandatory for accurate quantum chemistry (QC). SEC is particularly difficult to calculate and hence a qualitative understanding is important to judge the applicability of approximate QC methods. Existing scalar SEC diagnostics, however, lack the important information where the SEC effects occur in a molecule. We introduce an analysis tool based on a fractional occupation number weighted electron density (ρ(FOD) ) that is plotted in 3D for a pre-defined contour surface value. The scalar field is obtained by finite-temperature DFT calculations with pre-defined electronic temperature (e.g. TPSS at 5000 K). FOD plots only show the contribution of the "hot" (strongly correlated) electrons. We discuss illustrative plots for a broad range of chemical systems from small molecules to large conjugated molecules with polyradicaloid character. Spatial integration yields a single number which can be used to globally quantify SEC.
authors.affiliation: Universität Bonn, Mulliken Center for Theoretical Chemistry, Beringstrasse 4, Bonn, 53115 (Germany). grimme@thch.uni-bonn.de.
authors.forename: Stefan
authors.initials: S
authors.lastname: Grimme
authors.affiliation: Universität Bonn, Mulliken Center for Theoretical Chemistry, Beringstrasse 4, Bonn, 53115 (Germany).
authors.forename: Andreas
authors.initials: A
authors.lastname: Hansen
keywords: Fermi-smearing; biradicals; density functional theory; multi-reference diagnostic; non-dynamical correlation