Hi, I'm Yujin.
I want to find the workfunction for a slab with -1 charge using the CASTEP module.
If so, can we simply set -1 charge in the Calculation dialog and get it through potential analysis when the calculation is complete?
Would it make sense to do this?
Even if it is not CASTEP, is it meaningful to find the workfunction for -1 charge system?
When I tested with the Al structure, the workfunction energy became smaller as the vacuum became longer. And it appeared as a negative number. (-0.669 eV, -1.005 eV..)
Is there anybody who can give you advice?
Dear Yujin,
Periodic simulation software like CASTEP (and similar programs, such as ABINIT, Quantum Espresso and VASP) simulates an infinite, periodic system. If you were to genuinely charge your unit cell then you would have an infinite array of charges, and the electrostatic energy would diverge; for this reason, whenever you add (or remove) electrons from your simulation cell, a uniform compensating background charge is added to keep the overall cell charge neutral.
The addition of the background charge means that the unit cell is still charge-neutral and so the total energy is well defined. Because the background charge is uniform, it does not affect the distribution of the electrons so you still get reasonable electronic density distribution. However, the actual energies themselves (including the Kohn-Sham eigenvalues) are shifted, because of the background potential.
For any properties which depend on the actual energies, you will need to correct them for the effect of the neutralising background. There is quite a lot of work on this in the literature, but to start with I recommend you look at the work of Makov and Payne:
Article Periodic boundary conditions in ab initio calculations
This has also been discussed several times on the CASTEP email support list; I recommend you sign up, if you haven't already:
https://www.jiscmail.ac.uk/cgi-bin/wa-jisc.exe?SUBED1=CASTEP&A=1
Hope that helps,
Phil Hasnip
(CASTEP developer)