(38e) What More Can U Do: Alternative Applications of DFT+U

DFT+U and related approaches remain a popular way to fix electron delocalization problems in practical exchange-correlation functional implementations of density functional theory.  In DFT+U, standard XC functionals are augmented with a Hubbard model term that can successfully localize electrons in the ground state in molecules and open gaps in extended systems, producing insulators where common XC functionals might incorrectly predict metallic behavior.  Nevertheless, ongoing challenges for DFT+U's ascendancy include the fact that the approach is often perceived and employed as an ad hoc, empirical correction.  Here, we explain some alternative approaches wherein we not only calculate U from linear response of the system we are studying but we also motivate chemically-based renormalizations of linear-response calculations to obtain more broadly applicable values of U. We also demonstrate how the U can be viewed as a tool for tuning eigenvalues generally, even in cases with only 2p electrons present. In this case, we use a +U term to correct eigenvalues in closely spaced tautomeric states of molecules, thereby also correctin energetic spacing. Fundamentally, there's more that U can do and much of the efficacy of this approach lies in its ability to tune properties in a chemically-derived manner.