A two-state model of van der Waals (VdW) interactions is presented which completes the elementary description of the interactions occurring between atoms and molecules. A weak VdW bond is formed between closed-shell atoms or molecules as soon as the small Pauli repulsion is offset in long range by attractive VdW interactions. These are distortion and dispersion interactions., related to the electric properties of the interacting molecules, and described as small second-order effects in terms of long range interaction integrals betas. The explicit forms of these betas are derived from classical electrostatics for atom-atom, atom-linear molecule dispersion. and atom-linear molecule induction. Comparison between C-6 dispersion and induction coefficients for simple homodimers shows that dispersion is, in general. the most important attractive VdW interaction in long range. (C) 2004 Elsevier B.V. All rights reserved.