Accurate Total Energies without Self-Consistency

We present a new way of calculating approximate but accurate total energies within the framework of density functional theory. Our technique is based on an expansion of the energy functional to second order and does not require self-consistent iterations of the total density. The functional can be minimized by using the same techniques as developed for variational density functional perturbation theory. The method is ideally suited to systems composed of weakly interacting fragments, but it can also be applied to semiconductors and insulators. We show the versatility of our approach in a variety of examples exhibiting different types of chemical bonding.