Flying ice cube

The Flying ice cube is an artificial situation encountered in molecular dynamics simulations. It is due to an incorrect equipartition of energy by the thermostat, in particular, by thermostats that implement some form of periodic velocity rescaling, such as the Bussi-Donadio-Parrinello thermostat. The net result is that an instability forms where the kinetic energy may be drained from some degrees of freedom and be incorrectly fed into others. A manifestation of this would be the kinetic energy from the high frequency bond vibrations and angle bending in a system composed of, say, flexible water molecules ending up in the zero frequency mode of the kinetic energy of the system as a whole (i.e. centre of mass translation); the molecular motions would become frozen, resulting in a flying ice cube. Note that despite the name, this situation is not limited to simulations of water.

Solutions

• Use a thermostat that reassigns the velocity distribution, rather than rescaling the actual velocities. (see: Andersen thermostat)
• Reduce the frequency of the velocity rescaling.
• Increase the temperature bath coupling parameter. (see: Berendsen thermostat)

References

Related reading

• Liu-Ming Yan, Chao Sun, Hui-Ting Liu "Opposite phenomenon to the flying ice cube in molecular dynamics simulations of flexible TIP3P water", Advances in Manufacturing 1 pp. 160-165 (2013)