get the workfunction of charged system

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?

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?