Systems of more than Two Molecules

In the region of negligible overlap, the three-body interactions can be calculated, just like the two-body ones, from imaginary frequency polarizabilities. This has been done, for example, by Doran78 for interaction of triplets of noble-gas atoms. He gives the coefficients for dipole-dipole-dipole, dipole-dipole-quadrupole, dipole-quadru-pole-quadrupole, quadrupole-quadrupole-quadrupole, and dipole-dipolc-octupole terms. 

Most of the other calculations which involve more than two molecules raise the question of what is meant by a molecule. A number of SCF calculations have been carried out on assemblies of small atoms and ions which do not form familiar chemical species but which nonetheless appear to have larger dissociation energies than would normally be thought of as intermolecular. These include HeH3+,91 XeH+,9a He H, and He H+ where n = 1—4.9S It is concluded that linear symmetric He2H+ is stable while He3H+ and He4H+ are not. 

Another calculation that is of interest, but difficult to classify, is that of Wilson et a/.94 on the Hz-- He system. They are primarily interested in obtaining a potential surface on which to study the role of He as a third body in hydrogen atom recombination reactions, so that they include in their study configurations in which the H- -H distance is as large as 5 a.u., where the H-H interaction is quite small. They find that even in a calculation which does not include intermolecular correlation there are quite long-range contributions to non-additivity. 

Martin101 has calculated the long-range dipole moment for systems of three identical atoms. For the H H H system he has used a pseudo-spectral perturbation method, while for He- He He he has used the closure approximation. These quantities could be experimentally accessible via a pressure-induced i.r. absorption whose intensity varied as the cube of the pressure. 

Another property which has been considered is the hyperfine shift in hydrogen. Fortune and Certain104 have calculated the effect on this quantity of the interaction with a helium atom at large distances and give an estimate of the R coefficient of the change. 

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