Van der Waals coefficients of atoms and molecules from a simple approximation for the polarizability

A simple and computationally efficient scheme to calculate approximate imaginary-frequency-dependent polarizability, hence asymptotic van der Waals coefficient, within density-functional theory is proposed. The dynamical dipolar polarizabilities of atoms and molecules are calculated starting from the Thomas-Fermi-von Weizsacker (TFvW) approximation for the independent electron kinetic energy functional. The van der Waals coefficients for a number of closed-shell ions and a few molecules are hence calculated and compared with available values obtained by fully first-principles calculations. The success in these test cases shows the potential of the proposed TFvW approximate response function in capturing the essence of long-range correlations and may give useful information for constructing a functional which naturally includes van der Waals interactions.