CONSTRAINT SATISFACTION IN LOCAL AND GRADIENT SUSCEPTIBILITY APPROXIMATIONS - APPLICATION TO A VAN-DER-WAALS DENSITY-FUNCTIONAL

We show how charge conservation and reciprocity can be built into loca l density or gradient approximations for density-density response func tions (susceptibilities). We apply these ideas to derive from first pr inciples a variant of the Rapcewicz-Ashcroft formula for the van der W aals interaction. We also discuss how improved formulas may be obtaine d.