Toward the description of van der Waals interactions within density functional theory

On the basis of the adiabatic connection formula we propose several approximations for the total correlation energy functional, which, in the limit of two separated neutral subsystems, correctly reproduce the van der Waals R−6 behavior. We have calculated the corresponding van der Waals coefficients as well as total correlation energies, thus demonstrating the feasibility of a "seamless" functional.