Many-Body Interactions in Mixed Systems

As the polarization term in EFP is non-additive, the EFP method captures a majority of many-body effects in H-bonded systems. By definition, the many-body energy is a difference between a total energy of a system and energies of all pairwise interactions. In polar complexes, the many-body interactions are predominantly of 

H-bond acceptor with respect to the water molecule, while the two benzenes build a polar H-tt bond with each other. As a result, a H-bonding cycle exists also in this trimer. On the other hand, in the second one-water-two-benzene complex WlB2b, the water donates H-bonds to both benzenes and the many-body polarization becomes destructive. 

Analysis of the many-body interactions in tetramers suggests that benzene is actively involved in a collective H-bonding network in these clusters as well. For example, as shown in Fig. 5.2, several energetically low-lying benzene-water tetramers prefer cyclic structures that are similar to the structure of the water tetramer, and exhibit significant stabilizing many-body energies. 

3 Role of Polarization Energy Increases from Dimers to Bulk 

Tert-butyl alcohol (TBA) is the largest monohydric alcohol fully solvated in water. A level of micro-heterogeneity and mixing in water-TBA systems is under debate [42-43]. EFP reliably predicts strengths of hydrogen bonding in various water-TBA dimers, as compared to the MP2/6-311-I— -G(d,p) calculations [35] (see Fig. 5.3). Intermolecular distances obtained by the EFP and MP2 methods differ by less than 0.1 A. Stabilities of the dimers by EFP are within 0.8 kcal/mol of the MP2 stabilities, with the largest error observed for the first (W-TBAa) water-TBA dimer. 

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