Preparing a POTCAR

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. Copy the templates to one file named POTCAR in the working directory. It is important to paste the templates in the same order as the elements are specified in the POSCAR, e.g., by entering

 cat ~/potpaw_PBE.64/C/POTCAR ~/potpaw_PBE.64/O_h/POTCAR > POTCAR

As explained in detail below, there are sometimes multiple templates for one element with subtle differences.

Tip: Generally opt for the standard, computationally cheapest POTCAR files, but test if the property of interest is sensitive to the choice of the pseudopotential.

Step-by-step instructions

Step 1: Select a POTCAR for a certain family of exchange-correlation (XC) functionals and version.

Tip: Amongst all available potentials, the standard choice is to use the latest GGA version based on LEXCH=PE.

All potentials are constructed based on solving the scalar relativistic Schrödinger equation for a reference system with a certain XC functional. 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. We recommend using the latest available potentials. Older versions are available to ensure reproducibility.

Step 2: Choose a POTCAR with a different atomic configuration, hard or GW variant.

Tip: The standard choice is to use the plain version without suffixes.

The suffix may correspond to a harder potential necessary to describe short bond lengths or a different number of valence electrons, e.g., required to describe magnetism, as well as higher accuracy for unoccupied states required for optical response and many-body perturbation theory. See choosing pseudopotentials.

Step 3: Combine the potentials.

VASP expects a single POTCAR file in the working directory containing all species present in the structure. The POTCAR files must be concatenated. The order of files must correspond to the order of the species in the POSCAR file. If species names are given in the POSCAR, and they do not match the order in the POTCAR, a warning is printed, but VASP will still run. The order given in the POTCAR will take precedence over the order in POSCAR! You can copy the POTCAR to the working directory if you have only one element in your structure.

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 TiCo$_{2}$Si

In this example, we want to prepare a POTCAR for a PBE calculation of ferromagnetic TiCo$_{2}$Si. We are interested in the size of the magnetic moments.

The structure is defined by the following POSCAR:

TiCo2Si
 1.0
 -2.8580789844367893   -2.8580789844367889    0.0000000000000000
 -2.8580789844367889    0.0000000000000000   -2.8580789844367889
 -0.0000000000000005   -2.8580789844367889   -2.8580789844367889
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
  0.4999999999999999    0.5000000000000000    0.5000000000000000 Ti

We will use the potpaw_PBE.64 potential set, and since we are interested in magnetic properties, we should use potentials with additional semicore-states in the valence. 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/Ti_sv/POTCAR ~/potpaw_PBE.64/Si/POTCAR > ~/scratch/TiCo2Si/POTCAR

Related tags and sections

Available potentials, POTCAR, Choosing pseudopotentials, Theory:Pseudopotential basics, Projector-augmented-wave formalism