Long range three-body forces between helium and hydrogen atoms

The non-additive contribution to the long-range interaction energy of three atoms A, B and C at large distances Rab , Rbc and Rca apart may be written E(ABC)=γ(ABC)(1 + 3 cos ϑ a cos ϑ b cos ϑ c )/(RabRacRbc )e, where ϑ a , ϑ b and ϑ c are the internal angles of triangle ABC. The values of γ(ABC) for triads selected from hydrogen and helium atoms are calculated. In atomic units they are respectively 21·64, 8·12, 3·80 and 1·49 for the mixtures H-H-H, H-H-He, H-He-He and He-He-He.