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
additional tags to set	LANGEVIN_GAMMA,LANGEVIN_GAMMA_L,PMASS

The additional tags in the column for the thermostat don't have to be set necessarily because VASP has a set of default values for these tags. To use the NpT ensemble VASP has to be compiled with the precompiler flag -Dtbdyn. The 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 = 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.

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