MANY-BODY DISPERSION FORCES OF POLARIZABLE CLUSTERS AND LIQUIDS

A system of atoms with embedded Drude dispersion oscillators interacting through dipole-dipole forces is simulated. Using path integrals it is shown that after the coordinates of the dispersion oscillators are integrated out, the atoms interact through many-body dispersion forces to all orders of the dipole-dipole interaction. Simulations are carried out on clusters to see if the presence of many-body forces leads to ground state geometries different from those predicted from two-body potentials. In addition, the polarizability tensor of clusters is determined as a function of cluster size. Simulations are also carried out for fluids to see how many-body forces affect the pair correlation function. Lastly, the long-range interaction between van der Waals clusters is compared with the predictions of a summation over site-site two-body interactions. It is found that many-body forces have only a minor effect on the low energy geometries of van der Waals clusters, a somewhat surprising result given that many-body forces do give an important contribution to surface free energies of clusters and liquids. The vibrational frequencies of the breathing mode decrease by approximately 10%.