Direct approximation of the long- and short-range components of the exchange-correlation Kohn-Sham potential

An approximation scheme was developed for the Kohn-Sham exchange-correlation potential vxcσ, making use of a partitioning of vxcσ into a long-range screening vscrσ and a short-range response vresp component. For the response part, a model vmodrespσ was used, which represents vresp as weighted orbital density contributions, the weights being determined by the orbital energies. vmodrespσ possesses the proper short-range behavior and the atomic-shell stepped structure characteristic for vresp. For the screening part, two model potentials vmodscrσ were used, one with the accurate Slater potential; the other one with the generalized gradient approximation (GGA) for the exchange part. Both use the GGA for the Coulomb correlation contribution to vscrσ. The scheme provides an adequate approximation to vxcσ in the outer-valence region with both the proper asymptotics and a rather accurate estimate of the ionization potential from the highest one-electron energy and a reasonable estimate of atomic Exc and total energies Etot.