The long-range interaction of three atoms

A simple method proposed by the author for evaluating the van der Waals constant has been extended to give the non-additive third-order perturbation energy for three interacting atoms separated by large distances. For three hydrogen atoms in their ground states, separated by distances R 1 , R 2 and R 3 , the leading term has been calculated as 0 (3) = (43.2854±0.0020)Ω R 1 -3 R 2 -3 R 3 -3 a 0 9 ryd where Ω is an angular factor. This result compares favourably with the previous most accurate estimate 0 (3) = 42.8Ω R 1 -3 R 2 -3 R 3 -3 a 0 9 ryd obtained by Midzuno and Kihara.