Preparing a POTCAR: Difference between revisions

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The {{FILE|POTCAR}} is a VASP input file which holds the pseudopotentials and needs to be present in the execution directory for VASP to run.
The {{FILE|POTCAR}} file is a mandatory [[Input files|input file]] that holds the [[:Category:Pseudopotentials|pseudopotential]] for each element in the structure. The templates for each element can be downloaded from the [https://www.vasp.at/sign_in/portal/ VASP Portal].
There are sometimes multiple templates for one element with subtle differences.


==Step-by-step instructions==
==Step-by-step instructions==


'''Step 1:''' Select a POTCAR family.
'''Step 1:''' Select the [[Available_pseudopotentials|latest version]] of {{FILE|POTCAR}} files unless you need to use an older one to reproduce a result.


We recommend the use of the latest available potentials, currently the potpaw_LDA.64 and potpaw_PBE.64 sets, which can be downloaded from the VASP Portal. For compatibility reasons or to reproduce older calculations, it might be necessary to use another set, e.g. potpaw_LDA.54 or potpaw_GGA. All available potentials, grouped by their families, are listed in [[Construction:Available_PAW_potentials|Available_PAW_potentials]].
'''Step 2:''' Choose 
* [[Available_pseudopotentials#Standard_potentials|standard potentials]] for calculations depending mainly on occupied states, e.g., within density-functional theory, using [[hybrid functionals]], or
* [[Available_pseudopotentials#GW_potentials|GW variants]] if the calculation requires high accuracy for unoccupied states, i.e., for [[optical properties|optical response]] and [[many-body perturbation theory]].


'''Step 2:''': Choose a pseudopotential variant.
'''Step 3:''' Select a {{FILE|POTCAR}} for a certain family of [[Exchange-correlation functionals|exchange-correlation (XC) functionals]].
{{NB|tip|The standard choice is to use the [[Available_pseudopotentials#Standard_potentials|PBE version]] based on {{TAG|LEXCH}}{{=}}PE which has a high transferability to other [[XC functionals]].|:}}
:All potentials are constructed based on solving the scalar relativistic Schrödinger equation for a reference system with a certain [[Exchange-correlation functionals|XC functional]]. In most versions, one set is available for the LDA, and one for the GGA. The transferability to other [[Exchange-correlation functionals|XC functionals]] is seamless by specifying the {{TAG|XC}} tag in the {{FILE|INCAR}}.


Depending on your system, the property you want to calculate, and the method you are using different potential variants might be optimal. This choice is often non-trivial and some testing might be required. See [[Construction:Choosing_a_pseudopotential_variant]].
'''Step 4 (optional):''' Choose a [[Available pseudopotentials#Different variants specified by the suffix|different variant (reference atomic valence configuration, etc.) specified by the suffix]].  
{{NB|tip|The standard choice is to use the bold version in the [[Available pseudopotentials#Recommended PAW potentials|list of PAW potentials]].|:}}
:See [[choosing pseudopotentials]].
{{NB|important|Generally opt for the recommended {{FILE|POTCAR}} files, but test if the property of interest is sensitive to the choice of the pseudopotential. It may be possible to choose a computationally cheaper version or necessary to select a more demanding one.|:}}


'''Step 3:''' Combine the potentials.
'''Step 5:''' For a single element in the structure, you can copy the {{FILE|POTCAR}} to the working directory, e.g,
  cp /path/to/pot/Al/POTCAR .
:For structures with multiple elements, the selected {{FILE|POTCAR}} files must be concatenated to create one {{FILE|POTCAR}} file containing all species present in the structure. Combine the potentials by entering, for instance,
  cat /path/to/pot/Al/POTCAR /path/to/pot/C/POTCAR /path/to/pot/H/POTCAR > POTCAR


VASP expects a single POTCAR file in the working directory, containing all species present in the structure. To combine e.g. potpaw_PBE.64 potentials for titanium (with the ''3p'' states in the valence), carbon, and hydrogen on a UNIX machine, you might type the line:
:The order of the potentials must correspond to the order of the species in the {{FILE|POSCAR}} file.
{{NB|tip|If species names are given in the {{FILE|POSCAR}}, and the order does not match the order in the {{FILE|POTCAR}}, a warning is printed, but VASP will still run.|:}}


  cat ~/potpaw_PBE.64/Ti_pv/POTCAR ~/potpaw_PBE.64/C/POTCAR ~/potpaw_PBE.64/H/POTCAR > ~/your/working/dir/POTCAR
==Recommendations and advice==
 
{{NB|important|Except for the 1st-row elements, all PAW potentials are designed to work at an energy cutoff ({{TAG|ENMAX}} tag in the {{FILE|POTCAR}}) of roughly 250 eV. This is a key aspect of making the calculation computationally cheap. We recommend performing a convergence study of the quantity of interest with respect to the energy cutoff ({{TAG|ENCUT}} tag in the {{FILE|INCAR}}).}}  
{{NB|mind| Mismatched order of species in the {{FILE|POSCAR}} and {{FILE|POTCAR}} files is a common mistake! Add species names to your {{FILE|POSCAR}} to receive a warning if this happens.}}
{{NB|mind|You can mix and match {{FILE|POTCAR}} families. Even combining pseudopotentials generated with different [[XC functionals]] is possible, however make sure to specify the [[XC functional]] in the {{FILE|INCAR}}, see {{TAG|XC}}.}}
 
==Example for preparing a {{FILE|POTCAR}} for the Heusler alloy TiCo<sub>2</sub>Si==


The first file will correspond to the first species in the {{FILE|POSCAR}}. If species names are given in the {{FILE|POSCAR}}, and they do not match the order in the {{FILE|POTCAR}}, a warning is printed, but VASP will still run. The order given in the {{FILE|POTCAR}} will take precedence over the order in {{FILE|POSCAR}}!
In this example, we want to prepare a {{FILE|POTCAR}} for a PBE calculation of ferromagnetic TiCo<sub>2</sub>Si. We are interested in the energy difference between the ferromagnetic and the nonmagnetic solutions.
If you have only one element in your structure, you can just copy the {{FILE|POTCAR}}.


==Recommendations and advice==
The structure is defined by the following {{FILE|POSCAR}}:
TiCo2Si
  1.0
  0.0000000000000000    2.8580789844367893    2.8580789844367893
  2.8580789844367893    0.0000000000000000    2.8580789844367893
  2.8580789844367893    2.8580789844367893    0.0000000000000000
Co Si Ti
  2  1  1
direct
  0.7500000000000000    0.7500000000000000    0.7500000000000000 Co
  0.2500000000000000    0.2500000000000000    0.2500000000000000 Co
  0.0000000000000000    0.0000000000000000    0.0000000000000000 Si
  0.5000000000000000    0.5000000000000000    0.5000000000000000 Ti
 
We will use the potpaw_PBE.64 potential set, and since we are interested in small energy differences caused by different magnetic solutions, we should use potentials with additional semicore-states in the valence for the 3d metals. The Co_pv and Ti_sv potentials seem appropriate for the transition metals. We do not expect Si to become magnetic and are not interested in unoccupied states, so the Si potential is a good choice compared to the harder, computationally more demanding Si_GW or even Si_sv_GW.
 
On a UNIX machine, one can use the <code>cat</code> command to concatenate files together. One can redirect the output from <code>stdout</code> to a file using the <code>></code> operator. The order in the {{FILE|POSCAR}} dictates the order in the {{FILE|POTCAR}}:
cat ~/potpaw_PBE.64/Co_pv/POTCAR ~/potpaw_PBE.64/Si/POTCAR ~/potpaw_PBE.64/Ti_sv/POTCAR > ~/scratch/TiCo2Si/POTCAR
 
==Related tags and sections==


[[Available potentials]], {{FILE|POTCAR}}, [[Choosing pseudopotentials]], <!--[[Theory:Pseudopotential basics]],--> [[Projector-augmented-wave formalism]]


{{NB|mind| Mismatched order of species in the {{FILE|POSCAR}} and {{FILE|POTCAR}} files is a common mistake!}}
[[Category:Pseudopotentials]][[Category:Howto]]

Latest revision as of 13:26, 26 June 2024

The POTCAR file is a mandatory input file that holds the pseudopotential for each element in the structure. The templates for each element can be downloaded from the VASP Portal. There are sometimes multiple templates for one element with subtle differences.

Step-by-step instructions

Step 1: Select the latest version of POTCAR files unless you need to use an older one to reproduce a result.

Step 2: Choose

Step 3: Select a POTCAR for a certain family of exchange-correlation (XC) functionals.

Tip: The standard choice is to use the PBE version based on LEXCH=PE which has a high transferability to other XC functionals.
All potentials are constructed based on solving the scalar relativistic Schrödinger equation for a reference system with a certain XC functional. In most versions, one set is available for the LDA, and one for the GGA. The transferability to other XC functionals is seamless by specifying the XC tag in the INCAR.

Step 4 (optional): Choose a different variant (reference atomic valence configuration, etc.) specified by the suffix.

Tip: The standard choice is to use the bold version in the list of PAW potentials.
See choosing pseudopotentials.
Important: Generally opt for the recommended POTCAR files, but test if the property of interest is sensitive to the choice of the pseudopotential. It may be possible to choose a computationally cheaper version or necessary to select a more demanding one.

Step 5: For a single element in the structure, you can copy the POTCAR to the working directory, e.g,

 cp /path/to/pot/Al/POTCAR .
For structures with multiple elements, the selected POTCAR files must be concatenated to create one POTCAR file containing all species present in the structure. Combine the potentials by entering, for instance,
 cat /path/to/pot/Al/POTCAR /path/to/pot/C/POTCAR /path/to/pot/H/POTCAR > POTCAR 
The order of the potentials must correspond to the order of the species in the POSCAR file.
Tip: If species names are given in the POSCAR, and the order does not match the order in the POTCAR, a warning is printed, but VASP will still run.

Recommendations and advice

Important: Except for the 1st-row elements, all PAW potentials are designed to work at an energy cutoff (ENMAX tag in the POTCAR) of roughly 250 eV. This is a key aspect of making the calculation computationally cheap. We recommend performing a convergence study of the quantity of interest with respect to the energy cutoff (ENCUT tag in the INCAR).
Mind: Mismatched order of species in the POSCAR and POTCAR files is a common mistake! Add species names to your POSCAR to receive a warning if this happens.
Mind: You can mix and match POTCAR families. Even combining pseudopotentials generated with different XC functionals is possible, however make sure to specify the XC functional in the INCAR, see XC.

Example for preparing a POTCAR for the Heusler alloy TiCo2Si

In this example, we want to prepare a POTCAR for a PBE calculation of ferromagnetic TiCo2Si. We are interested in the energy difference between the ferromagnetic and the nonmagnetic solutions.

The structure is defined by the following POSCAR:

TiCo2Si
 1.0
  0.0000000000000000    2.8580789844367893    2.8580789844367893
  2.8580789844367893    0.0000000000000000    2.8580789844367893
  2.8580789844367893    2.8580789844367893    0.0000000000000000
Co Si Ti
 2  1  1
direct
  0.7500000000000000    0.7500000000000000    0.7500000000000000 Co
  0.2500000000000000    0.2500000000000000    0.2500000000000000 Co
  0.0000000000000000    0.0000000000000000    0.0000000000000000 Si
  0.5000000000000000    0.5000000000000000    0.5000000000000000 Ti

We will use the potpaw_PBE.64 potential set, and since we are interested in small energy differences caused by different magnetic solutions, we should use potentials with additional semicore-states in the valence for the 3d metals. The Co_pv and Ti_sv potentials seem appropriate for the transition metals. We do not expect Si to become magnetic and are not interested in unoccupied states, so the Si potential is a good choice compared to the harder, computationally more demanding Si_GW or even Si_sv_GW.

On a UNIX machine, one can use the cat command to concatenate files together. One can redirect the output from stdout to a file using the > operator. The order in the POSCAR dictates the order in the POTCAR:

cat ~/potpaw_PBE.64/Co_pv/POTCAR ~/potpaw_PBE.64/Si/POTCAR ~/potpaw_PBE.64/Ti_sv/POTCAR > ~/scratch/TiCo2Si/POTCAR

Related tags and sections

Available potentials, POTCAR, Choosing pseudopotentials, Projector-augmented-wave formalism