Intermolecular Interaction in Water Hexamer

The origin of the intermolecular interaction, especially the many-body interaction, in eight low-lying water hexamer structures (prism, cage, book-1, book-2, cyclic-chair, bag, cyclic-boat-1, and cyclic-boat-2) is unraveled using the localized molecular orbital energy decomposition analysis (LMO-EDA) method at the second-order Moller-Plesset perturbation (MP2) level of theory with a large basis set. It is found that the relative stabilities of these hexamer structures are determined by delicate balances between different types of interaction. According to LMO-EDA, electrostatic and exchange interactions are strictly pairwise additive. Dispersion interaction in these water hexamer structures is almost pairwise additive, with many-body effects varying from -0.13 to +0.05 kcal/mol. Repulsion interaction is roughly pairwise additive, with many-body effects varying from -0.84 to -0.62 kcal/mol. Polarization interaction is not pairwise additive, with many-body effects varying from -13.10 to -8.85 kcal/mol.