Accurate Molecular Van Der Waals Interactions from Ground-State Electron Density and Free-Atom Reference Data

We present a parameter-free method for an accurate determination of long-range van der Waals interactions from mean-field electronic structure calculations. Our method relies on the summation of interatomic C₆ coefficients, derived from the electron density of a molecule or solid and accurate reference data for the free atoms. The mean absolute error in the C₆ coefficients is 5.5% when compared to accurate experimental values for 1225 intermolecular pairs, irrespective of the employed exchange-correlation functional. We show that the effective atomic C₆ coefficients depend strongly on the bonding environment of an atom in a molecule. Finally, we analyze the van der Waals radii and the damping function in the C₆R^-6 correction method for density-functional theory calculations.