Many-body interactions in physisorption: Nonadditivity of film and substrate contributions to the holding potential

Calculations are presented of non-pair-additive contributions to the interaction energy of a probe atom with a physisorbed monolayer film and substrate. The Axilrod-Teller-Muto triple dipole energy is summed for triplets composed of probe atom, monolayer atom, and substrate atom. The lateral average, over probe atom positions, of the sum vanishes. A similar result and a nonvanishing term from a higher many-body interaction are derived also from the McLachlan theory of the dispersion interaction of two atoms in the presence of a continuum substrate.