  The density functional theory (DFT) interaction energy of a dimer is
rigorously derived from the monomer densities. To this end, the supermolecular
energy bifunctional is formulated in terms of mutually orthogonal sets of
orbitals of the constituent monomers. The orthogonality condition is preserved
in the solution of the Kohn-Sham equations through the Pauli blockade method.
Numerical implementation of the method provides interaction energies which
agree with those obtained from standard supermolecular calculations within less
than 0.1% error for three example functionals: Slater-Dirac, PBE0 and B3LYP,
and for two model van der Waals dimers: Ne2 and (C2H4)2, and two model H-bond
complexes: (HF)2 and (NH3)2.
