charge density difference calculation using VASP

[arwa68]

Hi,
I have two questions,
I am doing charge density difference calculation using VASP, I got the results but I am not sure about what the isovalue refers to after subtraction, my understanding is that the negative isovalue means gain electrons and the positive value indicates holes (lost electrons). I think it also depends on how one do the subtraction, i.e if I have AB system and to do charge density calculation I subtract ( AB -A -B) , then the negative isovalue refer to electrons, and positive refer to holes. is this correct or it is the opposite??

Another point is that the normal (before subtraction) CHGCAR file have always positive values (which refer to electron density), however, for the interface system I noticed that there are some negative values in my CHGCAR (not too much) noting that this appears in the CHGCAR before doing any subtraction. I don't know what the negative value here means, noting that in the normal supercell without interface I don't have any negative values. Can you please inform me why I got these negative values?

Thanks for your help

[graeme]

The subtraction scheme that you describe looks correct.

There can be small negative values in systems with vacuum. This is due to numerical issues related to projecting plane waves onto a real-space grid.