[CPMD-list] Average electrostatic potential (ELPOT, cpmd2cube)

[Audrius Alkauskas]

Dear CPMD'ers

I have a short question.  I have found a related question on the list, 
but it was not answered.

My question is related to an electrostatic potential which can be 
written to a file ELPOT. I converted it to a real space grid
with cpmd2cube and calculated an average over xy plane (I perform slab 
calculations). Results seemed reasonable, work functions
of metals were reproduced and comparable to the reports in the 
literature (for instance, 4.47eV for Ag(111), etc.). However, not untill 
recently did I notice that I don't understand
the output. In fact, if the total charge is electrons + gaussian 
distributions of positive charge on the nuclei, the average 
electrostatic potential should be zero,
and in my case it oscilates in the slab and becomes constant in the 
vacuum, but always negative (!). I know that it can be shifted by and 
arbitrary value.
So, my question: which exactly potential is written in the ELPOT: 
electrostatic, etc.? How it's mean value is defined?


Audrius