A Novel Constraint for the Simplified Description of Dispersion Forces

We propose a novel use of an exact constraint in the construction of simple approximations for response functions of interacting many-electron systems. Within its simplest local version, the resulting theory gives improved approximations for static atomic dipolar polarisabilities without the direct use of wavefunctions or semi-empirical cutoffs. It leads to correct van der Waals energies between distant planar systems, but over-corrects existing cutoff theories for the van der Waals C 6 coefficient for atoms. It is argued that a nonlocal-response version of the constrained theory will do better.