NpH ensemble
The NpH ensemble (isoenthalpic–isobaric ensemble) is a statistical ensemble that is used to study material properties under the conditions of a constant particle number N, a pressure p fluctuating around an equilibrium pressure p and a enthalpy H fluctuating around an equilibrium value H. This page describes how to sample the NpH ensemble from a molecular-dynamics run.
Instructions for setting up a NpH ensemble
To run a NpH molecular-dynamics simulation the Langevin thermostat has to be used. The LANGEVIN_GAMMA and LANGEVIN_GAMMA_L have to be zero because otherwise a NpT ensemble will be simulated. By setting the tag LANGEVIN_GAMMA=0 the friction term and the stochastic term of the Langevin thermostat will be zero, such that the velocities are determined by the Hellmann-Feynman forces or Machine-learned force fields only. Setting the tag LANGEVIN_GAMMA_L=0, removes the stochastic term and the friction term from the barostat, resulting in a box update depending solely on the kinetic stress tensor.
| NpH ensemble | Langevin |
|---|---|
| MDALGO | 3 |
| ISIF | 3 |
| LANGEVIN_GAMMA_L | 0 |
| LANGEVIN_GAMMA | 0 |
It is recommended to equilibrate the system of interest with a NPT molecular-dynamics run before starting the NpH run. A general guide for molecular-dynamics simulations can be found on the molecular-dynamics page.
An example INCAR file for the NpH ensemble
#INCAR molecular-dynamics tags NVE ensemble IBRION = 0 # choose molecular-dynamics MDALGO = 3 # using Andersen thermostat ISIF = 3 # compute stress tensor but do not change box volume/shape TEBEG = 300 # set temperature NSW = 10000 # number of time steps POTIM = 1.0 # time step in femto seconds LANGEVIN_GAMMA = 0.0 0.0 # setting friction and stochastic term of Langevin thermostat zero LANGEVIN_GAMMA_L = 0.0 # setting friction and stochastic term of Langevin barostat zero
| Mind: This INCAR file only contains the parameters for the molecular-dynamics part. The electronic minimization or the machine learning tags have to be added. |