What is the reference point of vaccum level when doing a workfunction calculation via VASP? and are there any differences from that at infinite location?
You consider the location furthest away from either surface for the vacuum energy level. Typically you choose 5-10A of vacuum on both sides to ensure that the electrostatic potential is flat. Also, don't forget to use dipole correction along the c-axis (especially if your slab is not symmetric in c). The work function is then, as already mentioned, the difference between the vacuum energy and the Fermi energy.
Peter Schindler
Thank you for your reply. Do you mean the 'either surface' is the top or bottom surface in our slab model?
Ge Liangbing
I meant both surfaces. If your slab is not mirror-symmetric (in c-direction) then you may get a different work function for the top surface than for the bottom surface.
Here is what you need to do to calculate the vacuum reference energy and the corresponding work-function energy using VASP:
1. Add LVTOT=.TRUE. (and to be more precise, also LVHAR=.TRUE.) to the INCAR file
2. Run the program with a POSCAR containing a sufficiently large vacuum space.
3. In the output you get a file called LOCPOT, enabling you to plot planar-averaged potential of the slab along its surface normal.
4. From this plot, you can obtain the vacuum reference energy Ev, corresponding to the potential at the center of the vacuum.
5. Having Ev, you can then calculate work-function energy as Ew=Ev - EF, where EF is the slab's Fermi energy given in OUTCAR.
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