LVACPOTAV: Difference between revisions

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'''LVACPOTAV''' = .TRUE. | .FALSE.  
{{TAGDEF|LVACPOTAV|.True. {{!}} .False.|.False.}}


Default: '''LVACPOTAV''' = .FALSE.  
Description: Switch on determining the vacuum potential by averaging the local potential over a field-free region.  
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Description: Determine the vacuum potential by averaging the local potential over a field-free region
{{TAG|LVACPOTAV}} 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 searching regions that are field-free ({{TAG|VACPOTFLAT}}), and the 2D-averaged charge density is nearly zero. The averaging is done in the direction of {{TAG|IDIPOL}} and is reported as the vacuum potential in the {{FILE|OUTCAR}}.
{{NB|tip| As {{TAG|LVACPOTAV}} performs a post-processing step, you may use it together with {{TAG|ALGO}} {{=}} None by restarting a converged calculation.}}
The vacuum potential is one of the quantities needed to [[Computing the work function|compute the work function]]. It can be extracted from the {{FILE|OUTCAR}} by the following bash command


This tag switches on the computation of the vacuum potential; i.e. the average of the local potential in the vacuum region.
  grep upper OUTCAR
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.
{{NB|warning| Note that '''LVACPOTAV''' is currently implemented for {{TAG|IDIPOL}} between 1 and 3.}}
{{NB|tip| As this tag performs a post-processing step to the calculation, you may use it together with {{TAG|ALGO}} {{=}} None for a quick look at the vacuum potentials after a converged calculation.}}
 
The vacuum potential is one of the quantities needed to compute the workfunction (see [[Computing the work function|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
 
<code>grep upper OUTCAR</code>
 
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.
{{NB|tip| In case you would like to determine the workfunction, we suggest using '''LVACPOTAV''' 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.}}


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, one might be more interesting than the other.
{{NB|tip|For determining the work function, we suggest using {{TAG|LVACPOTAV}} along with the {{TAG|LVHAR}} tag such that only the sum of the Hartree and ionic potentials are used in the calculation of the vacuum potential. This is because the exchange-correlation potential might be noisy in the vacuum region but should, in principle, be zero.}}
{{NB|mind|{{TAG|LVACPOTAV}} is available only for versions after 6.4.3.}}
Before VASP 6.4.3, the default algorithm reports the slice-averaged potential of a few grid points on both sides of the minimum 2D-averaged charge density in all directions (regardless of {{TAG|IDIPOL}}).
=== Use in conjunction with the dipole correction ===
=== 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.
A typical use case for {{TAG|LVACPOTAV}} is together with the dipole correction (including tags {{TAG|LDIPOL}} and {{TAG|IDIPOL}}). Switching on the dipole correction is crucial for determining the vacuum potential; without it, there will be no field-free region for dipolar systems.
{{NB|mind| Note that {{TAG|LVACPOTAV}} is currently implemented for {{TAG|IDIPOL}} between 1 and 3.}}


=== Warnings ===
=== Warnings ===
In case '''LVACPOTAV''' is not able to generate an accurate workfunction, the following warnings may be found in the [[OUTCAR]] file.  
In case {{TAG|LVACPOTAV}} is not able to generate an accurate work function, the following warnings may be found in the {{FILE|OUTCAR}} file.  


==== Vacuum region is likely too small ====
==== Vacuum region is likely too small ====
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Possible solutions include:
Possible solutions include:


* Making sure you have a large enough vacuum dimension
* Making sure you have a large enough vacuum dimension.
* Increasing the precision of your calculation by changing [[EDIFF]]
* Increasing the precision of your calculation by changing {{TAG|EDIFF}}.
{{NB|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 {{TAG|IDIPOL}} a few grid points on either direction of the minimum 2D-averaged charge density.}}


== Related tags and articles ==
== Related tags and articles ==
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{{TAG|LDIPOL}},
{{TAG|LDIPOL}},
{{TAG|IDIPOL}},
{{TAG|IDIPOL}},
{{TAG|VACPOTFLAT}}
{{TAG|VACPOTFLAT}},
{{TAG|WRT_POTENTIAL}},
{{TAG|LVTOT}},
{{TAG|LVHAR}}


[[Category:Electrostatics]][[Category:INCAR tag]]
[[Category:Electrostatics]][[Category:INCAR tag]]

Revision as of 10:43, 6 February 2024

LVACPOTAV = .True. | .False.
Default: LVACPOTAV = .False. 

Description: Switch on determining the vacuum potential by averaging the local potential over a field-free region.

LVACPOTAV 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 searching regions that are field-free (VACPOTFLAT), and the 2D-averaged charge density is nearly zero. The averaging is done in the direction of IDIPOL and is reported as the vacuum potential in the OUTCAR.

Tip: As LVACPOTAV performs a post-processing step, you may use it together with ALGO = None by restarting a converged calculation.

The vacuum potential is one of the quantities needed to compute the work function. It can be extracted from the OUTCAR by the following bash command 

 grep upper OUTCAR

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, one might be more interesting than the other.

Tip: For determining the work function, we suggest using LVACPOTAV 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. This is because the exchange-correlation potential might be noisy in the vacuum region but should, in principle, be zero.
Mind: LVACPOTAV is available only for versions after 6.4.3.

Before VASP 6.4.3, the default algorithm reports the slice-averaged potential of a few grid points on both sides of the minimum 2D-averaged charge density in all directions (regardless of IDIPOL).

Use in conjunction with the dipole correction

A typical use case for LVACPOTAV is together with the dipole correction (including tags LDIPOL and IDIPOL). Switching on the dipole correction is crucial for determining the vacuum potential; without it, there will be no field-free region for dipolar systems.

Mind: Note that LVACPOTAV is currently implemented for IDIPOL between 1 and 3.

Warnings

In case LVACPOTAV is not able to generate an accurate work function, the following warnings may be found in the OUTCAR file.

Vacuum region is likely too small

|     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

|     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.

Related tags and articles

DIPOL, LDIPOL, IDIPOL, VACPOTFLAT, WRT_POTENTIAL, LVTOT, LVHAR

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