Improving the exchange and correlation potential in density functional approximations through constraints

TL;DR

This paper discusses improving density functional approximations by imposing and relaxing constraints on the Kohn-Sham potential, leading to better asymptotic behavior and derivative discontinuity, which are crucial for accurate band-gap predictions.

Contribution

It introduces a relaxed positivity constraint in the constrained minimization of the effective potential, simplifying computations while maintaining accuracy in systems with more electrons.

Findings

Relaxing the positivity constraint reduces computational cost.

Constraining the screening charge improves asymptotic behavior of potentials.

The method captures the derivative discontinuity essential for accurate band-gap predictions.

Abstract

We review and expand on our work to impose constraints on the effective Kohn Sham (KS) potential of local and semi-local density functional approximations. In this work, we relax a previously imposed positivity constraint, which increased the computational cost and we find that it is safe to do so, except in systems with very few electrons. The constrained minimisation leads invariably to the solution of an optimised effective potential (OEP) equation in order to determine the KS potential. We review briefly our previous work on this problem and demonstrate with numerous examples that despite well-known mathematical issues of the OEP with finite basis sets, our OEP equations are well behaved. We demonstrate that constraining the screening charge of the Hartree, exchange and correlation potential not only corrects its asymptotic behaviour but also allows the exchange and correlation potential to exhibit nonzero derivative discontinuity, a feature of the exact KS potential that is necessary for the accurate prediction of band-gaps in solids but very hard to capture with semi-local approximations.