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This is a phenomenon that occurs when doing a molecular dynamics simulation of a molecule or protein. In order to model a closed system, you can set the parameters of your simulation so that the total energy remains constant. One would expect that this would maintain your molecule at a constant temperature. However, the vibrational energy within a molecule may sometimes be converted to translational energy. That is to say, the energy associated with internal wiggling of the molecule transforms into motion of the entire molecule through space.

Once the center of mass of a molecule begins to move, you have catastrophic cooling of the molecule, causing it to wiggle less and move faster. Eventually you have a molecule with zero internal energy (hence an ice cube) flying across the screen at alarming speed! If you use periodic boundary conditions, then the molecule will high-tail out of your boundary space, only to appear on the other side and shoot across again (similar to gunning the engine in Asteroids). This can be eliminated by fixing the center of mass of the system so that it cannot move.

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