LVACPOTAV
VACPOTAV = .TRUE. | .FALSE.
Default: VACPOTAV = .FALSE.
Description: Determine the vacuum potential by averaging the local potential over a field-free region
This tag switches on the computation of the vacuum potential; i.e. the average of the local potential in the vacuum region. It computes the average potential by iterating over regions of the 2D-averaged charge density (averaging done in the direction of IDIPOL) and tries to find a region in the cell where the charge density is nearly zero (corresponding to a field-free region in a cell without any compensating background charge). The potential is averaged over this region and is reported as the vacuum potential.
| Warning: This tag is available only for versions after 6.4.3. Prior to this version, the default algorithm used to determine the vacuum potential is used. The default algorithm simply reports the slice-averaged potential in all directions apart from IDIPOL a few grid points on either direction of the minimum 2D-averaged charge density. Note that VACPOTAV is currently implemented for IDIPOL between 1 and 3. |
The vacuum potential is one of the quantities needed to compute the workfunction (see here for a step-by-step guide on how to compute the workfunction with VASP). The vacuum potentials are written to the OUTCAR file and can be accessed with the following bash command
grep upper OUTCAR
which will produce the vacuum potentials. Note that two vacuum potentials will be produced, one corresponding to the upper termination of the slab and one corresponding to the lower. Depending on the system you are studying, one might be more interesting than another.
| Tip: In case you would like to determine the workfunction, we suggest using VACPOTAV along with the LVHAR tag such that only the sum of the Hartree and Ionic potentials are used in the calculation of the vacuum potential. |
Use in conjunction with the dipole correction
A typical use case for this tag is together with the dipole correction (including tags LDIPOL and IDIPOL). Switching on the dipole correction is crucial for determining the vacuum potential; without using it there will be no field free region for dipolar systems.
Warnings
In case VACPOTAV is not able to generate an accurate workfunction, the following warnings may be found in the OUTCAR file.
Vacuum region is likely too small
----------------------------------------------------------------------------- | | | W W AA RRRRR N N II N N GGGG !!! | | W W A A R R NN N II NN N G G !!! | | W W A A R R N N N II N N N G !!! | | W WW W AAAAAA RRRRR N N N II N N N G GGG ! | | WW WW A A R R N NN II N NN G G | | W W A A R R N N II N N GGGG !!! | | | | Did not find any points to average over, which means that no vacuum | | field-free region was found. Please increase the size of your cell | | in the dimension of the dipole correction to obtain accurate | | workfunction values. | | | -----------------------------------------------------------------------------
A possible solution to this problem is to increase the size of the vacuum dimension in your cell.
The minimum charge density in your cell may be too large
----------------------------------------------------------------------------- | | | W W AA RRRRR N N II N N GGGG !!! | | W W A A R R NN N II NN N G G !!! | | W W A A R R N N N II N N N G !!! | | W WW W AAAAAA RRRRR N N N II N N N G GGG ! | | WW WW A A R R N NN II N NN G G | | W W A A R R N N II N N GGGG !!! | | | | The minimum charge density times volume of the cell along the axis | | of the dipole correction is larger 1E-1, which could mean that your | | workfunction is not accurate as there is no field free region in | | your cell. Please consider either increasing the size of your cell | | along the dipole correction (vacuum dimension) or perhaps | | increasing the precision of your calculation. | | | -----------------------------------------------------------------------------
Possible solutions include:
- Making sure you have a large enough vacuum dimension
- Increasing the precision of your calculation by changing EDIFF