NpT ensemble: Difference between revisions

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The [[NpT ensemble]] (isothermal-isobaric ensemble) is a [[:Category:Ensembles|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 p and a temperature T fluctuating around an equilibrium value T. This page describes how to sample the NpT ensemble from a [[Molecular dynamics calculations|molecular-dynamics]] run.
The [[NpT ensemble]] (isothermal-isobaric ensemble) is a [[:Category:Ensembles|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 <math>\langle  p \rangle </math> and a temperature T fluctuating around an equilibrium value <math>\langle  T \rangle </math>. This page describes how to sample the NpT ensemble from a [[Molecular dynamics calculations|molecular-dynamics]] run.
   
   
''' Instructions for setting up a NpT ensemble '''
''' Instructions for setting up an NpT ensemble '''


The [[Langevin thermostat]] is the algorithm of choice when setting up a NpT [[Molecular dynamics calculations|molecular-dynamics]] run. To use the [[Langevin thermostat]] for a NpT simulation
The Parinello-Rahman algorithm{{cite|parrinello:prl:1980}}{{cite|parrinello:jap:1981}} is the method of choice when setting up an NpT [[Molecular dynamics calculations|molecular-dynamics]] run. To use the Parinello-Rahman algorithm the [[Langevin thermostat]] has
the {{TAG|ISIF}} tag has to be set to 3. Otherwise the lattice is not allowed to change
to be adjusted for an NpT simulation by setting the {{TAG|ISIF}}=3 in the {{FILE|INCAR}} file. Otherwise, the lattice is not allowed to change during the simulation, preventing VASP from keeping the pressure constant.
during the simulation, preventing VASP from keeping the pressure constant.
Additionally the user can set {{TAG|LANGEVIN_GAMMA}} as when simulating a [[NVT ensemble]],
Additionally the user can set {{TAG|LANGEVIN_GAMMA}} as when simulating a [[NVT ensemble]],
the tag {{TAG|LANGEVIN_GAMMA_L}} which is a friction coefficient for
the tag {{TAG|LANGEVIN_GAMMA_L}} which is a friction coefficient for
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|style="text-align:center;"| {{TAG|ISIF}}            ||  style="text-align:center;"|  3
|style="text-align:center;"| {{TAG|ISIF}}            ||  style="text-align:center;"|  3
|-  
|-  
| additional tags  to set || style="text-align:center;"| {{TAG|LANGEVIN_GAMMA}},{{TAG|LANGEVIN_GAMMA_L}},{{TAG|PMASS}}
| additional tags  to set || style="text-align:center;"| {{TAG|LANGEVIN_GAMMA}}, {{TAG|LANGEVIN_GAMMA_L}}
|-
| optional tags  to set || style="text-align:center;"| {{TAG|PMASS}}
|}
|}


The additional tags in the column for the thermostat don't have to be set necessarily because VASP
The additional tags in the column for the thermostat have to be set because the default
has a set of default values for these tags. To use the NpT ensemble VASP has to be compiled with the precompiler flag [[Precompiler_flags|-Dtbdyn]]. The A general guide for molecular-dynamics simulations can be found on the [[Molecular dynamics calculations|molecular-dynamics]] page.
values are zero resulting in a different ensemble. To use the NpT ensemble VASP has to be compiled with the precompiler flag [[Precompiler_flags|-Dtbdyn]]. 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]]''  
''An example {{FILE|INCAR}} file for the NpT ensemble''  


   #INCAR molecular-dynamics tags NVE ensemble  
   #INCAR molecular-dynamics tags NpT ensemble  
   {{TAGBL|IBRION}} = 0                      # choose molecular-dynamics  
   {{TAGBL|IBRION}} = 0                      # choose molecular-dynamics  
   {{TAGBL|MDALGO}} = 3                      # using Langevin thermostat
   {{TAGBL|MDALGO}} = 3                      # using Langevin thermostat
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   {{TAGBL|LANGEVIN_GAMMA}} = 10.0 10.0 10.0 # Langevin friction coefficient for three atomic species
   {{TAGBL|LANGEVIN_GAMMA}} = 10.0 10.0 10.0 # Langevin friction coefficient for three atomic species
   {{TAGBL|LANGEVIN_GAMMA_L}} = 10.0        # Langevin friction coefficient for lattice degrees of freedom
   {{TAGBL|LANGEVIN_GAMMA_L}} = 10.0        # Langevin friction coefficient for lattice degrees of freedom
  {{TAGBL|LANGEVIN_GAMMA_L}} = 10.0        # Langevin friction coefficient fir three atomic species
   {{TAGBL|PMASS}} = 1000                    # the fictitious mass of the lattice degrees of freedom
   {{TAGBL|PMASS}} = 1000                    # the fictitious mass of the lattice degrees of freedom
{{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.}}
{{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.}}
{{NB|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 {{FILE|ICONST}} file containing constraints for the Bravais lattice.}}


==Related tags and articles==
[[Molecular dynamics calculations|Molecular-dynamics calculations]], {{TAG|ISIF}}, {{TAG|MDALGO}}, {{TAG|LANGEVIN_GAMMA}}, {{TAG|LANGEVIN_GAMMA_L}}, {{TAG|PMASS}}, [[:Category:Ensembles|Ensembles]], {{FILE|ICONST}}


==Related tags and articles==
== References ==
[[Molecular dynamics calculations|Molecular-dynamics calculations]], {{TAG|ISIF}}, {{TAG|MDALGO}}, {{TAG|LANGEVIN_GAMMA}}, {{TAG|LANGEVIN_GAMMA_L}}, {{TAG|PMASS}}, [[:Category:Ensembles|Ensembles]]
<references/>


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

Latest revision as of 11:39, 24 April 2023

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[1][2] 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.

NpT ensemble Langevin
MDALGO 3
ISIF 3
additional tags to set LANGEVIN_GAMMA, LANGEVIN_GAMMA_L
optional tags to set PMASS

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.

Related tags and articles

Molecular-dynamics calculations, ISIF, MDALGO, LANGEVIN_GAMMA, LANGEVIN_GAMMA_L, PMASS, Ensembles, ICONST

References

  1. ↑ M. Parrinello and A. Rahman, Phys. Rev. Lett. 45, 1196 (1980).
  2. ↑ M. Parrinello and A. Rahman, J. Appl. Phys. 52, 7182 (1981).
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