Helium trimer

A well-known prototype of three-body interaction is the helium trimer [25-28]. The three-body interaction energy is discussed in detail in the above-mentioned works. It is interesting that the results, certainly due to the different approaches used, are not uniform. As demonstrated this by one example, the three-body term is found to be negative at the correlated level for a linear He-trimer (in ref.28.) while other authors (ref 27.) obtained a positive value for the same term. 

The many-body SAPT approach was used in Ref. [87] to compute the nonadditive interactions in the helium trimer. The computed points were fitted by a global potential function. The SAPT potential agrees well with accurate literature calculations at selected... 

The expressions for a general n-mer are also in their paper. °° Results for the helium trimer and preliminary results for the tetramer appear to be well be-... 

Other kinds of molecules besides 1 (which has unusual bond stereochemistry) for which these methods might fail to give good results are hypercoordinate molecules like NF5, molecules with noble gas atoms, particularly those of helium and neon, molecules with highly twisted C=C bonds, extraordinarily crowded molecules like hexaphenylethane, unknown dimers, trimers etc. of small familiar molecules, like C202 and N6, and very highly strained molecules. All these cases are discussed in a book on exotic molecules [4],... 

Continuing with HCN clusters as a structural motif, it was concluded that the larger clusters beyond the trimer were cyclic and due to rapid predissociation exhibited broad unresolvable spectra. However, using superfluid helium clusters as a growth medium for HCN clusters, linear structures have been observed for (HCN)4-7. While lacking in rotational detml, it is possible by merely counting to be able to assign the spectra to specific cluster sizes, as shown below in Fig. 15. 

Unit 6 The helium dimer and trimer (van der Waals interactions, three body effects)... 

Accurate ab initio calculations for the hehum dimer and trimer can be used in several other ways in thermal physics. One can use theoretical predictions to calibrate instruments designed to measure the density and dielectric virial coefficients, viscosity, thermal conductivity, speed of sound, and other properties of gases based on comparisons of theory with experiment for helium. 

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