NVT ensemble
The NVT 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 T. This page describes how to sample the NVT ensemble from a molecular dynamics run.
Instructions for setting up a NVT ensemble
There are four choices of thermostats which can either be stochastic or deterministic to simulate the NVT ensemble. The stochastic Andersen thermostat or Langevin thermostat, the deterministic Nose-Hoover thermostat or Multiple Andersen thermostats can be used. See table for the corresponding MDALGO 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 doing static optimization calculations with IBRION=1 or 2 and setting ISIF>2.
Other tags related to molecular dynamics simulations can be found here.
An example INCAR file for the Langevin thermostat could look like
#INCAR molecular dynamics tags NVT ensemble IBRION = 0 # choose molecular dynamics MDALGO = 3 # 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 LANGEVIN_GAMMA = 10.0 10.0 # setting friction coefficient in inverse time units for two atom types
| 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. |