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[[:Category: NVT ensemble]]                                                                                                                      
The [[NVT ensemble]] (canonical ensemble) is a [[:Category:Ensembles|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 <math>\langle T \rangle</math>.
This page describes how to sample the NVT ensemble from a [[Molecular dynamics calculations|molecular-dynamics]] run. 
   
   
== NVT ensemble ==
''' Instructions for setting up an 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 calculations|molecular dynamics]] run. 
''' Instructions for setting up a NVT ensemble '''  


There are 4 choices of thermostats which can either be stochastic or deterministic to simulate the NVT ensemble.  
There are four choices of thermostats to control the temperature for the NVT ensemble:  
The stochastic [[Andersen thermostat]] or [[Langevin thermostat]],  
The stochastic [[Andersen thermostat]] and [[Langevin thermostat]], as well as
the deterministic [[Nose-Hoover thermostat]] or [[MDALGO#MDALGO.3D13:_Multiple_Anderson_thermostats|Multiple Anderson thermostats]] can be used.  
the deterministic [[Nose-Hoover thermostat]] and [[MDALGO#MDALGO.3D13:_Multiple_Anderson_thermostats|Multiple Andersen thermostats]] can be used.  
See table for the corresponding {{TAG|MDALGO}} tags.  
See table for the corresponding {{TAG|MDALGO}} setting and related tags.  


 
{|class="wikitable" style="margin:aut
{|class="wikitable" style="margin:aut  
! NVT ensemble !! Andersen !! Langevin !! Nose-Hoover !! Multiple Andersen   
! NVT ensemble !! Andersen !! Langevin !! Nose-Hoover !! Multiple Andersen   
|-  
|-  
|MDALGO           ||     1        ||     3            ||     2      ||       13  
|{{TAG|MDALGO}}          || style="text-align:center;"|   1        ||   style="text-align:center;"|  3            ||   style="text-align:center;"|    0 or 2      ||   style="text-align:center;"|      13  
|- {{TAG|PSUBSYS}  
|- {{TAG|PSUBSYS}}  
| additional tags || {{TAG|ANDERSEN_PROB}} ||  {{TAG|LANGEVIN_GAMMA}} || {{TAG|SMASS}}      || {{TAG|NSUBSYS}}, {{TAG|TSUBSYS}}, {{TAG|PSUBSYS}}  
| additional tags to set || style="text-align:center;"| {{TAG|ANDERSEN_PROB}} || style="text-align:center;"|  {{TAG|LANGEVIN_GAMMA}} || style="text-align:center;"| {{TAG|SMASS}}      || style="text-align:center;"|  {{TAG|NSUBSYS}}, {{TAG|TSUBSYS}}, {{TAG|PSUBSYS}}  
|}  
|}  


The user is not allowed to set ISF>2 because then the volume of the simulation would be allowed to change
The additional tags in the column for every thermostat have to be set. For example, the [[Nose-Hoover thermostat]] needs the additional {{TAG|SMASS}} tag. 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]].
and the simulation would not result in a NVT ensemble.  
To enforce constant volume throughout the calculation, {{TAG|ISIF}} has to be set to less than three. The cell shape and volume have
Other flags related to molecular dynamics simulations can be found [[Molecular dynamics calculations|here]].  
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.
''A simple example INCAR file for the [[Langevin thermostat]] could look like'''


  #INCAR molecular dynamics tags NVT ensemble
''Example {{FILE|INCAR}} file for the [[Nose-Hoover thermostat]]''
  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


Note this {{FILE|INCAR}} file only contains the parameters for the molecular dynamics part. The
  #INCAR molecular-dynamics tags NVT ensemble
[[Electronic minimization|electronic minimization]] or the [[Machine-learned force fields|machine learning]] tags  
  {{TAGBL|IBRION}} = 0                  # choose molecular dynamics
have to be added.
  {{TAGBL|MDALGO}} = 2                  # using Langevin thermostat
  {{TAGBL|ISIF}} = 2                    # compute stress tensor but do not change box volume/shape
  {{TAGBL|TEBEG}} = 300                  # set temperature
  {{TAGBL|NSW}} = 10000                  # number of time steps
  {{TAGBL|POTIM}} = 1.0                  # time step in femto seconds
  {{TAGBL|SMASS}} = 1.0                  # setting the virtual mass for the Nose-Hoover thermostat
{{NB|mind| This {{FILE|INCAR}} file only contains the parameters for the molecular-dynamics part. The [[Electronic minimization|electronic minimization]] or the [[Machine-learned force fields|machine learning]] tags have to be added.}}


==Related tags and articles==
==Related tags and articles==
{{FILE|REPORT}}, [[Molecular dynamics calculations]]
[[Molecular dynamics calculations|Molecular-dynamics calculations]], {{TAG|ISIF}}, {{TAG|MDALGO}}, {{TAG|ISIF}}, {{TAG|MDALGO}}, {{TAG|LANGEVIN_GAMMA}}, {{TAG|SMASS}},{{TAG|ANDERSEN_PROB}}, {{TAG|NSUBSYS}}, {{TAG|TSUBSYS}}, {{TAG|PSUBSYS}}
 
 
 


[[Category:Molecular dynamics]][[Category:Ensembles]][[Category:Thermostats]]
[[Category:Molecular dynamics]][[Category:Ensembles]][[Category:Thermostats]]

Latest revision as of 11:45, 24 April 2023

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 . 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

Molecular-dynamics calculations, ISIF, MDALGO, ISIF, MDALGO, LANGEVIN_GAMMA, SMASS,ANDERSEN_PROB, NSUBSYS, TSUBSYS, PSUBSYS