Thermostats

The system may be coupled to a heat bath to ensure that the average system temperature is maintained close to the requested temperature, $T_{\rm ext}$. When this is done the equations of motion are modified and the system no longer samples the microcanonical ensemble. Instead trajectories in the canonical (NVT) ensemble, or something close to it are generated. DL_POLY_2 comes with three different thermostats: Nosé-Hoover [18], Berendsen [16], and Gaussian constraints [17]. Of these only the Nosé-Hoover algorithm generates trajectories in the canonical (NVT) ensemble. The other methods will produce properties that typically differ from canonical averages by ${\cal O}(1/{\cal N})$ [12]