ON THE APPLICABILITY OF THE SCREENED-COULOMB EXCHANGE MODEL IN KOHN-SHAM DENSITY-FUNCTIONAL STUDIES

The screened-Coulomb exchange model used first in the X alpha method a nd later on in Kohn-Sham density functional theory is reexamined. Base d on the well-elaborated framework of the local spin-density approxima tion, we show that this model is not well suited when systems with a f inite number of electrons are concerned, because it does not respect t he pair-density sum rule. A proper modification of the model is propos ed by reformulating it in terms of a screened-exchange here and ensuri ng the sum rule far this hole. As a result, it is shown how the static screened exchange in finite systems is accompanied by a conjugate ant iscreened one. The possible consequences of this effect on the applica tion of the screened-Coulomb exchange model are discussed. (C) 1994 Jo hn Wiley and Sons, Inc.