Andersen thermostat

In the approach proposed by Andersen the system is thermally coupled to a fictitious heat bath with the desired temperature. The coupling is represented by stochastic collisions that act occasionally on randomly selected particles. In particular the momentum of the lucky particle at every collision step is instantaneously chosen at random from the Boltzmann distribution at the selected temperature. The collision probability is defined as an average number of collisions per atom and time-step and the collision frequency occurs with the following distribution

$P(t)=\nu e^{-\nu t}.$

This quantity can be controlled by the flag ANDERSEN_PROB. The total number of collisions with the heat-bath is written in the file REPORT for each MD step.