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 | ''' Instructions for setting up an NpT ensemble ''' | ||
The | 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}} | 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}} | | 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 | 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 [[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 NpT ensemble'' | ''An example {{FILE|INCAR}} file for the NpT ensemble'' | ||
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{{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== | ==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]] | [[Molecular dynamics calculations|Molecular-dynamics calculations]], {{TAG|ISIF}}, {{TAG|MDALGO}}, {{TAG|LANGEVIN_GAMMA}}, {{TAG|LANGEVIN_GAMMA_L}}, {{TAG|PMASS}}, [[:Category:Ensembles|Ensembles]], {{FILE|ICONST}} | ||
== References == | |||
<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