New approximations for calculating dispersion coefficients

Improved approaches for finding approximate values of dispersion coefficients are proposed. They are based on scaling the values of time-dependent Hartree-Fock (TDHF) dispersion coefficients by factors that use the ratio of the estimated true value and the TDHF value of static dipole polarizabilities. It is shown that for a set of 14 atoms and molecules the average absolute-value deviation of the estimated two-body isotropic dispersion coefficients with respect to the dipole oscillator strength distribution results is smaller than 1.0% for two of our approaches. For three-body isotropic dispersion coefficients the corresponding deviations are smaller than 1.2%. Our approximations work particularly well compared to approximations proposed by other researchers in cases where the TDHF results differ from the reference values by more than 10%. (C) 2004 American Institute of Physics.