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In simulations of aqueous systems it is common to freeze the bond vibrationand angle bending modes in water to allow for a longer time-step $\delta t$ forintegrating the equations of motion. Thus $\delta t = 2$ fs is often used insimulating rigid models of water. We simulate the SPC/E model of water using$\delta t$ from 0.5 fs to 3.0 fs. We find that for all but $\delta = 0.5$ fs,equipartition between translational and rotational modes is violated: therotational modes are at a lower temperature than the translation modes. Theautocorrelation of the velocities corresponding to the respective modes showsthat the rotational relaxation occurs at a time-scale comparable to vibrationalperiods, invalidating the original assumption for freezing vibrations. $\deltat$ also influences thermodynamic properties: the mean system potential energiesare not converged until $\delta t = 0.5$ fs, and the excess entropy ofhydration of a soft, repulsive cavity is also sensitive to $\delta t$.

Equilibration between Translational and Rotational Modes in Molecular  Dynamics Simulations of Rigid Water Requires a Smaller Integration Time-Step  Than Often Used