Exchange-correlation energy and potential as approximate functionals of occupied and virtual Kohn–Sham orbitals: Application to dissociating H2

The standard local density approximation and generalized gradient approximations fail to properly describe the dissociation of an electron pair bond, yielding large errors ~on the order of 50 kcal/mol! at long bond distances. To remedy this failure, a self-consistent Kohn–Sham ~KS! method is proposed with the exchange-correlation ~xc! energy and potential depending on both occupied and virtual KS orbitals. The xc energy functional of Buijse and Baerends @Mol. Phys. 100, 401 ~2002!; Phys. Rev. Lett. 87, 133004 ~2001!# is employed, which, based on an ansatz for the xc-hole amplitude, is able to reproduce the important dynamical and nondynamical effects of Coulomb correlation through the efficient use of virtual orbitals. Self-consistent calculations require the corresponding xc potential to be obtained, to which end the optimized effective potential ~OEP! method is used within the common energy denominator approximation for the static orbital Green’s function. The problem of the asymptotic divergence of the xc potential of the OEP when a finite number of virtual orbitals is used is addressed. The self-consistent calculations reproduce very well the entire H2 potential curve, describing correctly the gradual buildup of strong left–right correlation in stretched H2 . © 2003 American Institute of Physics. @DOI: 10.1063/1.1562197#