Structure of ice Ih. A b i n i t i o two- and three-body water–water potentials and geometry optimization

Abstract

The OO distance in ice (2.76 Å) is much shorter than in water dimer (2.98 Å). No first principle potential function has successfully described the observed OO shrinkage. We have calculated water–water two-body interaction potentials with an ab initio MO method by varying not only the OO distance but also the OH distance. New analytical fits of two-body potential functions have been obtained. The nearest-neighbor three-body potential has been evaluated for proton-ordered ice–Ih structures. With ab initio one, two, and nearest-neighbor three-body potentials, ∠HOH fixed at the monomer value, we have been able to obtain ROO=2.79 Å, ROH=0.977 Å with the binding energy of 15.8 kcal/mol per H2O molecule for proton-ordered antiferroelectric ice Ih and ROO=2.85 Å, ROH=0.972 Å with the binding energy of 14.3 kcal/mol per molecule for ferroelectric ice Ih. The three-body interaction, aided by the two-body interaction, contributes to the OO shrinkage. Factors that would favor larger ROH stretch and ROO shrinkage have been discussed.