# Ensembles¶

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.