Currently the following ensembles are implemented in the ReaxFF program:

  • NVE
  • NVT with a Berendsen thermostat (see the pdf manual for details)
  • NVT with a Nose-Hoover chain (NHC) thermostat
  • NPT with a Berendsen baro- and thermostat (see the pdf manual for details)
  • Anderson-Hoover (AH) or Parrinello-Rahman-Hoover (PRH) NPT with NHC thermostats

Nose-Hoover chains NVT ensemble

Both the NHC-NVT and the AH-NPT are implemented following the paper by G.J. Martyna et al. [G J Martyna, M E Tuckerman, D J Tobias, M L Klein, Mol. Phys. 87 (1996) 1117]. The following parameters are used: N_c=5, N_ys=5, Nchains=10. The NHC-NVT ensemble is invoked by setting imdmet=2 in the control file. The tdamp control parameter (\(\tau_{t}\)) is used to determine the first thermostat mass as \(Q = N_{free} k T_{set} \tau_{t}^2\). It should be noted that the \(\tau_{t}\) in the Nose-Hoover method corresponds to the period of the characteristic oscillations of the system, which is different from the \(\tau\) in the Berendsen thermostat where it determines the relaxation time. Relaxation of the system with a Nose-Hoover thermostat usually takes longer than with the Berendsen one with the same \(\tau\).

The temperature regimes and zones specified via the tregime.in file can also be used with the NHC thermostat the same way as with the Berendsen one.

AH and PRH NPT ensemble with NHC

The Anderson-Hoover NPT ensemble is invoked by setting imdmet=9 in the control file. Everyting wrttien above about the NHC thermostat applies also to the thermostat part of the AH-NPT. The particle thermostat and the barostat are controlled by two different chains. The masses in the barostat thermostat chain are determined by the tdamp parameter as \(Q_{p} = k T_{set} \tau_{t}^2\). The pdamp control parameter \(\tau_{p}\) defines the barostat mass as \(W = (N_{free} + 3) k T_{set} \tau_{p}^2\). Similar to the NHC thermostat, the \(\tau_{p}\) here corresponds to the period of the characteristic pressure fluctuations of the system.

For an anisotrpic NPT dynamics one should specify imdmet=10 (fixed cell angles) or imdmet=11 (full cell fluctuations). It should be noted that the anisotrpic NPT dynamics has not been well tested yet and thus it should be considered experimental.