A molecular dynamics study of structural transitions in small water clusters in the presence of an external electric field

Abstract

The present work constitutes a thorough study of the response of a relatively small water cluster (N=32) to external static electric fields in the 0.5×107 to 108 V/cm range, at T=200 K. As the electric field is varied, the system undergoes a phase transition to structures resembling incomplete nanotubes consisting of stacked squares arranged perpendicularly to the field direction. For further field increase the system transforms continuously to more open structures, reminiscent of the proton ordered forms of cubic ice, found also in the liquid. Regarding the dynamic response of the cluster, this is reflected in a profound way on the nonmonotonic variation of the reorientational decay rates of the molecular intrinsic axes and of the self-diffusion coefficients along and perpendicular to the field lines. In general the external field induces a considerable increase of the reorientational decay rates of all axes, except for the strongest field where the electrofreezing effect is observed. Reorientational relaxation has been found to obey a stretched exponential behavior of the Kohlrausch–Williams–Watts-type, where a one-to-one correspondence between the β-exponent variation with the field, molecular cooperativity, and translational diffusion has been established.