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 run the Langevin thermostat has to be used.
| NpH ensemble | Langevin | |
|---|---|---|
| MDALGO | 3 | |
| ISIF | 3 | |
| LANGEVIN_GAMMA_L | 0 | |
| additional tags to set | ANDERSEN_PROB=0.0 | SMASS=-3 |
The additional tags in the column for every thermostat have to be set to the given values. Otherwise the NVE ensemble will not be realized. There are two implementations of the Nose-Hoover thermostat in VASP which will give the same results. The MDALGO=0 version can be used even if the code was compiled without the precompiler option -Dtbdyn. To enforce constant volume throughout the calculation, ISIF has to be set to less than three. The cell shape and volume have to be preoptimized when doing NVT simulations. This can either be done with a NPT molecular-dynamics run or by performing structure and volume optimization with IBRION=1 or 2 and setting ISIF>2. A general guide for molecular-dynamics simulations can be found on the molecular-dynamics page.
An example INCAR file for the Andersen thermostat
#INCAR molecular-dynamics tags NVE ensemble IBRION = 0 # choose molecular-dynamics MDALGO = 1 # using Andersen thermostat ISIF = 2 # 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 ANDERSEN_PROB = 0.0 # setting Andersen collision probability to zero to get NVE enseble
| 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. |