Two- and three-body dispersion forces with one excited molecule Original Research Article

The interaction between molecules and the Maxwell electromagnetic field is used to determine two- and three-body dispersion forces, taking into account retardation effects fully. The case in which one of the molecules is in an electronically excited state is contrasted with that in which all molecules are in their ground states. The very long-range, retarded, potentials involving excited molecules are dominated by the contributions arising from real photon exchange. For pair interaction energies the lead term behaves as R−2, for the three-body energies a modulated potential falling off as 1abc is predicted (a, b, c are the distances between pairs of molecules). However for small separations the lead terms arise from both real and virtual photon exchange. The resulting potentials are identical with those obtained from elementary perturbation theory with electrostatic couplings. Two approaches have been used to obtain these potentials. The first is based on a form of response theory involving the use of induced dipole moments and zero-point energy differences. In the second, the energy shifts are calculated directly using fourth-order perturbation theory taking into account the contribution of the poles which arise for systems with excited molecules.