A DENSITY-FUNCTIONAL THEORY STUDY OF FREQUENCY-DEPENDENT POLARIZABILITIES AND VAN-DER-WAALS DISPERSION COEFFICIENTS FOR POLYATOMIC-MOLECULES

A method for calculating frequency-dependent polarizabilities and Van der Waals dispersion coefficients, which scales favorably with the number of electrons, has been implemented in the Amsterdam Density Functional package. Time-dependent Density Functional Theory is used within the Adiabatic Local Density Approximation (ALDA). Contrary to earlier studies with this approximation, our implementation applies to arbitrary closed-shell molecular systems. Our results for the isotropic part of the Van der Waals dispersion energy are of comparable quality as those obtained in TDCHF calculations. The ALDA results for the relative anisotropy of the dipole dispersion energy compare favorably to TDCHF and MBPT results. Two semi-empirical ways to calculate the dispersion energy anisotropy are evaluated. Large bases which include diffuse functions are necessary for a good description of the frequency-dependent properties considered here. (C) 1995 American Institute of Physics.