NpH ensemble: Difference between revisions

Latest revision as of 10:38, 21 November 2023

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 $\langle p \rangle$ and a conserved enthalpy H (up to numerical inaccuracies). This page describes how to sample the NpH ensemble from a molecular-dynamics run.

Instructions for setting up a NpH ensemble

To run an NpH molecular-dynamics simulation MDALGO = 3 has to be used. The LANGEVIN_GAMMA and LANGEVIN_GAMMA_L have to be zero to disable any thermostatting. By setting the tag LANGEVIN_GAMMA = 0 the friction term and the stochastic term of the Langevin thermostat will be zero, such that the velocities are determined by the Hellmann-Feynman forces or machine-learned force fields only. Setting the tag LANGEVIN_GAMMA_L = 0, removes the stochastic term and the friction term from the barostat, resulting in a box update depending solely on the kinetic stress tensor. The inertia of lattice degrees-of-freedom is controlled with the PMASS tag.

NpH ensemble	Langevin
MDALGO	3
ISIF	3
LANGEVIN_GAMMA_L	0
LANGEVIN_GAMMA	0
optional tags to set	PMASS

It is recommended to equilibrate the system of interest with an NPT molecular-dynamics run before starting the NpH run. A general guide for molecular-dynamics simulations can be found on the molecular-dynamics page.

An example INCAR file for the NpH ensemble

 #INCAR molecular-dynamics tags NpH ensemble 
 IBRION = 0                   # choose molecular-dynamics 
 MDALGO = 3                   # using Andersen thermostat
 ISIF = 3                     # 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 = 0.0 0.0     # setting friction and stochastic term of Langevin thermostat zero
 LANGEVIN_GAMMA_L = 0.0       # setting friction and stochastic term of Langevin barostat zero

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 1: / Line 1: @@
-The [[NpH ensemble]] (isoenthalpic–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 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 calculations|molecular-dynamics]] run.
+The [[NpH ensemble]] (isoenthalpic–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 pressure <math> \langle p \rangle </math> and a conserved enthalpy H (up to numerical inaccuracies). This page describes how to sample the NpH ensemble from a [[Molecular dynamics calculations|molecular-dynamics]] run.
 ''' 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 an NpH [[Molecular dynamics calculations|molecular-dynamics]] simulation {{TAGO|MDALGO|3}} has to be used. The {{TAG|LANGEVIN_GAMMA}} and {{TAG|LANGEVIN_GAMMA_L}} have to be zero to disable any thermostatting. By setting the tag {{TAGO|LANGEVIN_GAMMA|0}} the friction term and
+the stochastic term of the [[Langevin thermostat]] will be zero, such that the velocities are determined by the Hellmann-Feynman forces or machine-learned force fields only. Setting the tag {{TAGO|LANGEVIN_GAMMA_L|0}},
+removes the stochastic term and the friction term from the barostat, resulting in a box update
+depending solely on the kinetic stress tensor. The inertia of lattice degrees-of-freedom is controlled with the {{TAG|PMASS}} tag.
 {|class="wikitable" style="margin:aut
@@ Line 14: / Line 17: @@
 |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
+|-
+| optional tags  to set || style="text-align:center;"| {{TAG|PMASS}}
 |}
-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 an [[NpT_ensemble|NPT]] [[Molecular dynamics calculations|molecular-dynamics]] run before starting the NpH 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]]''
+''An example {{FILE|INCAR}} file for the NpH ensemble''
-   #INCAR molecular-dynamics tags NVE ensemble
+   #INCAR molecular-dynamics tags NpH ensemble
    {{TAGBL|IBRION}} = 0                   # choose molecular-dynamics
-   {{TAGBL|MDALGO}} = 1                   # using Andersen thermostat
+   {{TAGBL|MDALGO}} = 3                   # using Andersen thermostat
-   {{TAGBL|ISIF}} = 2                     # compute stress tensor but do not change box volume/shape
+   {{TAGBL|ISIF}} = 3                     # 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|ANDERSEN_PROB}} = 0.0          # setting Andersen collision probability to zero to get NVE enseble
+   {{TAGBL|LANGEVIN_GAMMA}} = 0.0 0.0     # setting friction and stochastic term of Langevin thermostat zero
+  {{TAGBL|LANGEVIN_GAMMA_L}} = 0.0       # setting friction and stochastic term of Langevin barostat zero
 {{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==
-[[Molecular dynamics calculations|Molecular-dynamics calculations]], {{TAG|ISIF}}, {{TAG|MDALGO}}, [[:Category:Ensembles|Ensembles]]
+[[Molecular dynamics calculations|Molecular-dynamics calculations]], {{TAG|ISIF}}, {{TAG|MDALGO}}, {{TAG|LANGEVIN_GAMMA}}, {{TAG|LANGEVIN_GAMMA_L}}, [[:Category:Ensembles|Ensembles]]
 [[Category:Molecular dynamics]][[Category:Ensembles]][[Category:Thermostats]]