A Practicable Real‐Space Measure and Visualization of Static Electron‐Correlation Effects

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.