A simple method for the prediction of the orientation of H2O molecules in ionic crystals

Abstract

A robust and fast method is presented that provides a simple real-space convergent solution for identifying equilibrium orientations of crystallization H2O molecules in ionic crystals, on the basis of zero net torque. The predicted H2O orientations constrained by rotational equilibrium are compared with neutron scattering experiments and/or ab initio density functional theory (DFT) calculations. The comparison shows that predicted and observed H2O orientations are consistent, demonstrating the reliability of the reported torque method. Moreover, the rotational equilibrium conditions predict an alternative, not previously observed, H2O orientation in kernite [Na2B4O6(OH)2·3H2O], and this may explain anomalously large displacement parameters that have been reported for this mineral. Complementary DFT computations corroborate that the two orientations are geometrically distinct and energetically near degenerate.