The binding energy of the ring form of (H2O)6 is calculated by means of the MP2 and localized-orbital MP2 (LMP2) methods. The LMP2 method is found to be effective at reducing basis set superposition error in the electron correlation contribution to the binding energy. The inclusion of f and g functions on the O atoms and d and f functions on the H atoms leads to an increase of about 3.2 kcal/mol in the stability of the ring form of (H2O)6. Our best estimate of the binding energy is −44.3 kcal/mol. Of this, three-body interactions contribute −11.60 kcal/mol, and the four- , five- , and six-body interactions combined contribute −2.0 kcal/mol. Although inclusion of electron correlation energy is crucial for obtaining an accurate value of the two-body interactions, the net effect of electron correlation on the three- and higher-body interactions is only about 0.2 kcal/mol. Based on these results, a computationally efficient strategy for obtaining accurate binding energies of hydrogen-bonded clusters is proposed.

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