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 $\langle p \rangle$ and a temperature T fluctuating around an equilibrium value $\langle T \rangle$. This page describes how to sample the NpT ensemble from a molecular-dynamics run.

Instructions for setting up an NpT ensemble

The Parinello-Rahman algorithm is the method of choice when setting up an NpT molecular-dynamics run. To use the Parinello-Rahman algorithm the Langevin thermostat has to be adjusted for an NpT simulation by setting the ISIF=3 in the INCAR file. 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
additional tags to set	LANGEVIN_GAMMA, LANGEVIN_GAMMA_L
optional tags to set	PMASS

The additional tags in the column for the thermostat have to be set because the default values are zero resulting in a different ensemble. To use the NpT ensemble VASP has to be compiled with the precompiler flag -Dtbdyn. A general guide for molecular-dynamics simulations can be found on the molecular-dynamics page.

An example INCAR file for the NpT ensemble

 #INCAR molecular-dynamics tags NpT ensemble 
 IBRION = 0                      # choose molecular-dynamics 
 MDALGO = 3                      # using Langevin thermostat
 ISIF = 3                        # compute stress tensor and change box volume/shape 
 TEBEG = 300                     # set temperature 
 NSW = 10000                     # number of time steps 
 POTIM = 1.0                     # time step in femto seconds 
 LANGEVIN_GAMMA = 10.0 10.0 10.0 # Langevin friction coefficient for three atomic species
 LANGEVIN_GAMMA_L = 10.0         # Langevin friction coefficient for lattice degrees of freedom
 PMASS = 1000                    # the fictitious mass of the lattice degrees of freedom

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.

Calculations of systems with only short range order (e.g. liquids) may lead to irreversible deformations of the cell within this ensemble. In this cases one must use constraints for the Bravais lattice using an ICONST file.

Related tags and articles

Molecular-dynamics calculations, ISIF, MDALGO, LANGEVIN_GAMMA, LANGEVIN_GAMMA_L, PMASS, Ensembles