Density functional methods and the spatial distribution of electronic charge

Abstract

The effect of density functional (DFT) methods upon the form of the electronic charge ditribution is analyzed at the molecular and atomic levels by comparison of the perturbation these functionals make to the HF/DZP density distribution with the changes MP2 electron correlation produce. The density functional methods mimic MP2 densities near and within the valence shell regions, contracting charge towards the nucleus of heavy atoms. The outer distribution outside the valence regions, are not well mimicked, with MP2 polarizing charge away from the nucleus while the DFT techniques contract charge towards the nucleus. The DFT methods mimic the ‘MP2-like’ polarizations in the inner regions of atoms by pushing charge into areas with higher gradients of the charge density in order to lower the total energy of the systems as directed by the form of the functionals. In the outer regions, where there is no corresponding energetic motive, charge is not polarized away from the atoms. This would suggest that the DFT procedures would not be as successful as MP2 in predicting properties based upon the outer charge distribution of molecules, e.g., inter-molecular complexes and hyperpolarizabilities.