van der Waals Interactions in Density-Functional Theory

We present a framework for long-range density-functional theory which is valid for interactions between isolated fragments of matter at large separation. The van der Waals coefficients for interactions between a large number of pairs of atoms are calculated and compared to available first-principles calculations. The success in this test case shows a way of extending density-functional theory calculations with local or semilocal approximations to include van der Waals forces.