Effects of translational and rotational degrees of freedom on the properties of model water

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Molecular dynamics simulations with separate thermostats for rotational and translational motions were used to study the effects of these degrees of freedom on the structure of water at a fixed density. To describe water molecules, we used the SPC/E model. The results indicate that an increase of the rotational temperature, Tᵣ, causes a significant breaking of the hydrogen bonds. This is not the case, at least not to such an extent, when the translational temperature, Tₜ, is raised. The probability of finding an empty spherical cavity (no water molecule present) of a given size, strongly decreases with an increase of Tᵣ, but this only marginally affects the free energy of the hydrophobe insertion. The excess internal energy increases proportionally with an increase of Tᵣ, while an increase of Tₜ yields a much smaller effect at high temperatures. The diffusion coefficient of water exhibits a non-monotonous behaviour with an increase of the rotational temperature.