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Density functional approximations for the exchange-correlation energy Exc DFA of an electronic system are often improved by admixing some exact exchange Ex: Exc Exc DFA 1/n Ex Ex DFA. This procedure is justified when the error in Exc DFA arises from the 0 or exchange end of the coupling-constant integral 0 1 d Exc, DFA. We argue that the optimum integer n is approximately the lowest order of Görling–Levy perturbation theory which provides a realistic description of the coupling-constant dependence Exc, in the range 0 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 second-order perturbation theory with the generalized gradient approximation.© 1996 American Institute of Physics. S0021-96069601846-6

Kohn–Sham density functional theory1–3 typically makes a local or semilocal …