NVT ensemble

The NVT ensemble (canonical ensemble) is a statistical ensemble that is used to study material properties under the conditions of a constant particle number N, constant volume V and a temperature fluctuating around an equilibrium value $\langle T \rangle$ . This page describes how to sample the NVT ensemble from a molecular-dynamics run.

Instructions for setting up an NVT ensemble

There are four choices of thermostats to control the temperature for the NVT ensemble: The stochastic Andersen thermostat and Langevin thermostat, as well as the deterministic Nose-Hoover thermostat and Multiple Andersen thermostats can be used. See table for the corresponding MDALGO setting and related tags.

NVT ensemble	Andersen	Langevin	Nose-Hoover	Multiple Andersen
MDALGO	1	3	0 or 2	13
additional tags to set	ANDERSEN_PROB	LANGEVIN_GAMMA	SMASS	NSUBSYS, TSUBSYS, PSUBSYS

The additional tags in the column for every thermostat have to be set. For example, the Nose-Hoover thermostat needs the additional SMASS tag. 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.

Example INCAR file for the Nose-Hoover thermostat

 #INCAR molecular-dynamics tags NVT ensemble 
 IBRION = 0                   # choose molecular dynamics 
 MDALGO = 2                   # using Langevin 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 
 SMASS = 1.0                  # setting the virtual mass for the Nose-Hoover thermostat

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.

Related tags and articles