The time-dependent variation-perturbation method is applied to the estimation of van der Waals forces (-C/R6 term) in atoms and molecules. TJse is made of an expression for the coefficient C involving an integral over imaginary frequencies of the dynamic polarizabilities of the interacting species. With an analytic approximation to the dynamic polarizabilities that can be extended into the imaginary frequency range, a simple formula for the van der Waals force is obtained. Applications of the method to the interactions between S-state atoms (H, He, Ne, Ar) are given and the results are compared with available theoretical and experimental values. Also, atom-molecule interactions involving (H, H2) and (He, H 2) are examined, and the anisotropies in the van der Waals forces are calculated.