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 and a temperature T fluctuating around an equilibrium value . 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.
|additional tags to set
|optional tags to set
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.
|Warning: Calculations of systems with limited long-range order (e.g. liquids) may lead to irreversible deformations of the cell within this ensemble. For those systems one must use an ICONST file containing constraints for the Bravais lattice.