Ar2-Chromophore Clusters Exchange and Dispersion Nonadditivity

The three-body exchange nonadditivity, e ch, is conveniently categorized as related to two general mechanisms depicted in Fig.8. The first, where permutation of electrons encompasses all three monomers and the second, where only two are involved. They are referred to as 

An opposite situation arises for a collinear form. Orthogonalizing two monomers gets in the way of orthogonalizing the third monomer. Instead of cooperating, the monomers now compete. Eventually, the net repulsion is larger than the sum of all three pair repulsions. The nonadditive effect is repulsive. 

The mechanism of dispersion nonadditivity was proposed over 50 years ago by Axilrod and Teller [76] and independently by Muto [77]. It is referred to as the correlation of three instantaneous dipoles. To better appreciate the behavior of this term, let us consider the same two extreme configurations of a trimer as for the TE nonadditivity described earlier. In the equilateral triangle the three monomers cooperate in correlating with each other i.e. when a third monomer gets close, it sees the other two conveniently pre-correlated. In contrast, for the collinear approach of a third monomer this pre-correlation takes place in the wrong direction. Since pair dispersion interaction is attractive, the nonadditivity is repulsive for the equilateral trimer and attractive in the collinear form. 

The simplest model for the three-body dispersion energy is provided by the well-known triple-dipole expression 

The structure of this complex is shown in Fig. 9. Ab initio calculations of the three-body potential determined the relative importance of each term. They showed [78,79] that the total three-body interaction is very anisotropic with respect to the in-plane and out-of-plane rotations of HC1 within the cluster (see Fig. 10). It is instructive to have a closer look at the composition of this three-body term. 

---