Nonpairwise Interactions and Vacancy Formation Energies in Simple Molecular Solids

A method is given for summing all orders of nonpairwise contributions to the van der Waals interaction energy of a substitutional impurity in a monatomic molecular crystal using a Lorentz oscillator model as an approximation. The nonpairwise contribution to the energy of removing an atom from the lattice is obtained as a limiting case. Applied to rare-gas solids, the model suggests vacancy formation energies reduced from the two-body values, but insufficiently to give complete agreement with experiment.