NpT ensemble
The NpT ensemble (isothermal-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 value p and a temperature T fluctuating around an equilibrium value T. This page describes how to sample the NpT ensemble from a molecular-dynamics run.
Instructions for setting up a NpT ensemble
The Langevin thermostat is the algorithm of choice when setting up a NpT molecular-dynamics run. To use the Langevin thermostat for a NpT simulation the ISIF tag has to be set to 3. Otherwise the lattice is not allowed to change during the simulation, preventing VASP from keeping the pressure constant. Additionally the user can set LANGEVIN_GAMMA as when simulating a NVT ensemble, the tag LANGEVIN_GAMMA_L which is a friction coefficient for the lattice degrees of freedom and the PMASS tag to assign a fictitious mass to the lattice degrees of freedom.
| NpT ensemble | Langevin |
|---|---|
| MDALGO | 3 |
| ISIF | 3 |
| optional tags to set | LANGEVIN_GAMMA,LANGEVIN_GAMMA_L,PMASS |
The optional tags in the column for the thermostat don't have to be set necessarily because VASP has a set of default values. 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. |