In trod u ctionAs will be substantiated in this workshop, the knowledge of intermolecular potentials opens the way to (the calculation of) many observable properties, for microscopic as well as macroscopic systems. In the first category are thermodynamic stability, the spectra of Van der Waals molecules [1][2][3][4], an d molecular beam scattering cross sections [5][6][7], elastic or inelastic state-to-state, to tal or differential. In the second category are various bulk gas and condensed m atter properties. Measured gas phase properties [8,9] which depend directly on the intermolecular potential are virial coef ficients, viscosity and diffusion coefficients, therm al conductivity, sound absorption, pressure broadening of spectral lines, nuclear m agnetic relaxation and depolarized Rayleigh scattering. Additional information is obtained from the effects of electric and magnetic fields on the transport properties (Senftleben-Beenakker effects).
2stability and lattice vibrations of molecular solids [11]. On the other hand, all the measured data may be used, and have actually been used in several examples, to construct or improve (semi-)empirical intermolecular potentials.Several reviews on intermolecular potentials have appeared during the past five years [2,3,[12][13][14][15][16], and hence I shall simply outline the most important points. In teractions between molecules are usually divided into long-range interactions and short-range interactions. At long range, i.e. when the charge clouds of the interacting molecules do not overlap, the interaction energy can be obtained formally by standard Rayleigh-Schrödinger perturbation theory. The perturbation, which is the intermolecular interaction operator, can be expanded as a multipole series in powers of R~1, where R is the distance between th e centers-of-mass of the molecules. The first-order energy is the electrostatic multipole-multipole interaction energy. The second-order energy contains the induction (multipole-induced multipole) energy and the (nonclassical) dispersion energy. For molecular ions the electrostatic and induction in teractions are strongly dominant. For polar, e.g. hydrogen bonded, molecules the electrostatic interactions are still the most important contribution, while the induc tion and dispersion energies are comparable. For apolar molecules, i.e. molecules with small dipole moments, the dispersion energy becomes the most important (attractive) long range interaction. The long range interactions are completely determined by the permanent multipole moments and the, static as well as frequency-dependent, multi pole polarizabilities of the monomers.Since the molecular charge clouds have exponential tails, there is always some overlap between them. The effects of this overlap are twofold. Penetration causes the exact electrostatic interaction between continuous, overlapping charge clouds to deviate from its representation by a multipole series. This is correctly included in the Rayleigh-Schrodinger perturbation theory if one avoids the expansi...
