My Community

Dear VASP communcity,

I am very new to VASP and a bit confused about the naming used for potcar files. Please forgive me if my questions are too simple, but your comments would help me.

- PBE is in the GGA functional class; so why we have a potcar_PBE and potcar_GGA class? what is the difference here?

- In a few papers, I saw that they mention ‘The exchange correlation effect was described within the generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerh (PBE).’ If I want to use similar settings should I use the potcar_PBE files?

- And a more general question on vasp and cp2k. I used to believe that I can use one or the other. But I saw in a few papers that they have used both. May I ask why? What is it that we can do with one of the packages but not the other?

For example:
Mater. Res. Express 7 (2020) 065603 - https://doi.org/10.1088/2053-1591/ab98cd
‘Calculations of the structural optimization and the total energy based on a double-ζ polarization quality Gaussian basis sets (DZVP-MOLOPT-SR-GTH) of 500 Ry cut-off energy were performed with the freely available program package CP2K [28] using a mixed Gaussian and plane wave (GPW) approach [29]. The generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerh (PBE) [30] was used to describe the exchange correlation effect. And the DFT-D3 semi-empirical dispersion correction scheme was adopted due to the Van der Waals force (vdW) between graphyne and metals. A dipole correction was applied to avoid spurious interactions between periodic images of the slab. The electronic self-convergence criterion is set to 1×10−5 Ry and the forces on all the atoms are converged to within 4.5×10−4 Ry/Bohr with respect to ionic relaxation. Also, the electronic structure calculations based on the plane-wave basis sets of 450 eV cut-off energy were performed with the Vienna ab initio simulation package (VASP) [31]. The exchange correlation effect was described within the generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerh (PBE). Monkhorst–Pack scheme was employed to discretize the Brillouin-zone (BZ) integrations with a sampling density of 0.2/Å. The electronic self-convergence criterion is set to 1×10−4 eV.’

Thanks in advance for your help.

Best regards,
Azade YAZDAN

The difference between the potcar_GGA and potcar_PBE is the parametrization.

potcar_GGA uses PW91
https://link.springer.com/chapter/10.10 ... 9-0316-7_7

potcar_PBE uses PBE
https://journals.aps.org/prl/abstract/1 ... tt.77.3865

The choice of identifier is unfortunately misleading as indeed PBE is also a generalized gradient approximated (GGA) functional.
Note, however, that the potcar_GGAs are outdated and it is recommended that you use potentials from the potcar_PBE set.
For example, the potcar_GGA does not include information on the kinetic energy so when running a metaGGA calculation VASP might produce unreliable results.

Regarding your question about using cp2k or VASP I think you should find the answer in the paper you posted or by contacting the authors.
I am not an expert in cp2k but I believe it uses a mixed Gaussian and plane-wave basis.
VASP uses the PAW method.
As to using one or the other it really depends on the type of calculations you want to perform.