Rationale for mixing exact exchange with density functional approximations
Abstract
Density functional approximations for the exchangecorrelation energy E^{DFA}_{xc} of an electronic system are often improved by admixing some exact exchange E_{x}: E_{xc}≊E^{DFA}_{xc}+(1/n)(E_{x}E^{DFA}_{x}). This procedure is justified when the error in E^{DFA}_{xc} arises from the λ=0 or exchange end of the couplingconstant integral ∫^{1}_{0} dλ E^{DFA}_{xc,λ}. We argue that the optimum integer n is approximately the lowest order of GörlingLevy perturbation theory which provides a realistic description of the couplingconstant dependence E_{xc,λ} in the range 0≤λ≤1, whence n≊4 for atomization energies of typical molecules. We also propose a continuous generalization of n as an index of correlation strength, and a possible mixing of secondorder perturbation theory with the generalized gradient approximation.
 Publication:

Journal of Chemical Physics
 Pub Date:
 December 1996
 DOI:
 10.1063/1.472933
 Bibcode:
 1996JChPh.105.9982P