Mixing Exact Exchange with GGA: When to Say When

Abstract

The local spin density (LSD) approximation [1] has long been the method of choice for solid-state physics calculations. With the advent of generalized gradient approximations (GGAs) [2–7], density functional calculations for bond energies became an inexpensive alternative to traditional ab-initio quantum chemical calculations [8]. The recently derived PBE approximation [9] reduces the mean absolute error on a set of 20 small molecules from 31 kcal/mol in LSD to 8 kcal/mol. Both LSD and PBE approximations are non-empirical, in that all their parameters (other than the exchange-correlation energy per electron of a uniform gas) are fundamental constants. PBE is a simplification of the PW91 GGA [5–7].