NpH ensemble: Difference between revisions

Revision as of 13:19, 11 August 2022

The NpH ensemble (isoenthalpic–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 pressure p and a enthalpy H fluctuating around an equilibrium value H. This page describes how to sample the NpH ensemble from a molecular-dynamics run.

Instructions for setting up a NpH ensemble

To run a NpH molecular-dynamics run the Langevin thermostat has to be used. The LANGEVIN_GAMMA and LANGEVIN_GAMMA_L have to be zero because otherwise a NpT ensemble will simulated. By setting the tag LANGEVIN_GAMMA the friction term and the stochastic term of the Langevin thermostat thermostat will be zero, such that the velocities are determined by the Hellmann-Feynman forces or Machine-learned force fields only.

NpH ensemble	Langevin
MDALGO	3
ISIF	3
LANGEVIN_GAMMA_L	0
LANGEVIN_GAMMA	0

It is recommended to equilibrate the system of interest with a NPT molecular-dynamics run. 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 = 1                   # using Andersen 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 
 ANDERSEN_PROB = 0.0          # setting Andersen collision probability to zero to get NVE enseble

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.

@@ Line 3: / Line 3: @@
 ''' Instructions for setting up a NpH ensemble '''
-To run a NpH [[Molecular dynamics calculations|molecular-dynamics]] run the [[Langevin thermostat]] has to be used.
+To run a NpH [[Molecular dynamics calculations|molecular-dynamics]] run the [[Langevin thermostat]] has to be used. The {{TAG|LANGEVIN_GAMMA}} and {{TAG|LANGEVIN_GAMMA_L|}} have to be zero because otherwise a NpT
+ensemble will simulated. By setting the tag {{TAG|LANGEVIN_GAMMA}} the friction term and
+the stochastic term of the [[Langevin thermostat]] thermostat will be zero, such that the velocities are determined by the Hellmann-Feynman forces or Machine-learned force fields only.
 {|class="wikitable" style="margin:aut
@@ Line 14: / Line 16: @@
 |style="text-align:center;"| {{TAG|LANGEVIN_GAMMA_L}}           ||   style="text-align:center;"|   0
 |-
-| additional tags  to set || style="text-align:center;"| {{TAG|ANDERSEN_PROB}}=0.0 ||style="text-align:center;"| {{TAG|SMASS}}=-3
+|style="text-align:center;"| {{TAG|LANGEVIN_GAMMA}}           ||   style="text-align:center;"|   0
 |}
-The additional tags in the column for every thermostat have to be set to the given values. Otherwise the NVE ensemble will not be realized.
+It is recommended to equilibrate the system of interest with a [[NpT_ensemble|NPT]] [[Molecular dynamics calculations|molecular-dynamics]] run. A general guide for molecular-dynamics simulations can be found on the [[Molecular dynamics calculations|molecular-dynamics]] page.
-There are two implementations of the [[Nose-Hoover thermostat]] in VASP which will give the same results. The {{TAG|MDALGO}}=0 version can be used even if the code was compiled without the precompiler option [[Precompiler_options#-Dtbdyn|-Dtbdyn]]. To enforce constant volume throughout the calculation, {{TAG|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_ensemble|NPT]] [[Molecular dynamics calculations|molecular-dynamics]] run or by performing structure and volume optimization with {{TAG|IBRION}}=1 or 2 and setting {{TAG|ISIF}}>2.
-A general guide for molecular-dynamics simulations can be found on the [[Molecular dynamics calculations|molecular-dynamics]] page.
 ''An example {{FILE|INCAR}} file for the [[Andersen thermostat]]''

Revision as of 13:19, 11 August 2022

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