Effect of circular flow on single-molecule cross-mode correlations in liquid water

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Abstract
In a molecular-dynamics computer simulation of liquid water, a circular field of force is applied to a central region of about 250 molecules out of a total of 500 molecules. This is achieved by dividing the molecular-dynamics sample into an outer cube subject to periodic boundary conditions, and an inner cube, whose side is 0.75 of that of the outer cube. The inner cube (containing roughly half the molecules on average) can therefore be seen as subject to natural or thermal boundary conditions for the circular flow set up there. The effect of this circular flow on the molecular dynamics of all 500 molecules is then studied by means of autocorrelation and cross-correlation functions in the laboratory frame, and in the moving frame defined by the principal moment of inertia axes of the water molecule. The results are compared with equivalents in 500 molecules of liquid water at equilibrium, and the effects of rotational flow on the symmetry of various single-molecule cross-mode coupling tensors are discussed in detail.