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 local density or gradient approximations for density-density response functions (susceptibilities). We apply these ideas to derive from first principles a variant of the Rapcewicz-Ashcroft formula for the van der Waals interaction. We also discuss how improved formulas may be obtained. {copyright} {ital 1996 The American Physical Society.}