Molecular orbital calculation of several ices using the pseudolattice method

Molecular orbital (MO) studies of several ices (I_h, I_c, III, IX, VI) were made by using the pseudolattice method. According to the method water molecules in the ice model experience an identical environment without using periodic boundary conditions. It is found that the stability of ice I_h compared to I_c has its origin in the structural difference of the hydrogen bond parallel to the c axis. The difference in binding energy between proton-order and proton-disorder ice I_h is negligible (0.01 kcal/mol), and thus the residual entropoy of ice I_h becomes important for determining the proton orderliness. In ice III, the model in which hydrogen atoms are aligned on the O-O bond is energetically favorable. Linear hydrogen-bond models of ice III and ice IX cannot descriminate between the stability of ice III and ice IX. In ice VI, a linear hydrogen-bond model is considerably unstable because of the repulsive contact of non-hydrogen-bonded neighbors. For high-pressure ices having distorted hydrogen bonds, the positions of hydrogen atoms play an important role in determining the binding energies and charge distributions of the ices.