  We analyse a path to construct density functionals for the dispersion
interaction energy from an expression in terms of the ground state densities
and exchange-correlation holes of the isolated fragments. The expression is
based on a constrained search formalism for a supramolecular wavefunction that
is forced to leave the diagonal of the many-body density matrix of each
fragment unchanged, and is exact for the interaction between one-electron
densities. We discuss several aspects: the needed features a density functional
approximation for the exchange-correlation holes of the monomers should have,
the optimal choice of the one-electron basis needed (named "dispersals"), and
the functional derivative with respect to monomer density variations.
