Near-equilibrium trimers

We will now examine the major nonadditive components of interaction energy for four different near-equilibrium trimers. These components are displayed in Fig. 33.1. The first observation that can be made is that the ratio of the three-body component to the sum of two-body interactions changes dramatically from system to system from 0.4% for He3 to 16% for the water trimer. The total nonadditive contribution can also be of either sign, even for similar systems like rare gas trimers, cf. He3 v. Ar3. The increased role of nonadditive terms is clearly correlated with the polarity of a system the more polar a system is, the larger are the relative contributions of nonadditive terms with respect to two-body terms. 

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