Unmasking Static Correlation Error in Hybrid Kohn-Sham Density Functional Theory.

To uncover the way Hartree-Fock exchange brings static correlation error into hybrid Kohn-Sham density functional theory, we compare the potential energy curves of four diatomic molecules, namely, H2, F2, HF, and NaF, using both restricted and unrestricted Kohn-Sham theory. We find that increasing the percentage of Hartree-Fock exchange significantly increases the static correlation error because Hartree-Fock exchange replaces the ability of local exchange in density functionals to account for the localization effects associated with static correlation.