so far, there are two methods to calculate the charge density difference ,see following:
1. difference charge density: delta_RHO=RHO_sc - RHO_atom, where RHO_sc and RHO_atom charge density files can be calculated by setting the parameters ICHARG=2 and ICHGARG=12 respectively.
2.deformation charge density: supposing that the system AB was composed by A and B , the defomation charge density was avialable as depited in the formula: delta_charge=charge(AB)-charge(A)-charge(B).
my question is that what is the difference between two charge densities?
see the VCDD depicted in the picture, which charge density difference is it belong to ?
It's pretty hard to answer because you don't specify what you are asking. Your title is "how to calculate charge density difference?" but in the text you're asking "What is the difference between two charge densities?. Which is it?
Next, your definition of option 1 is very bad. "where RHO_sc and RHO_atom charge density files can be calculated by setting the parameters ICHARG=2 and ICHGARG=12 respectively." I find setting ICHARG and ICHGARG not to do a thing, because I'm probably not using the same program as you are.
Your options 1 and 2 are similar. If A and B are atoms, definitions 1 and 2 are in full agreement. The difference is that if A and B are molecules, then definition 1 gives the charge density difference.
You ask about a "VCDD", but don't give the definition for the acronym. You have also cut the definition out of the picture.
From what I can make out of the clip, it appears to say quite clearly that it's the difference between the molecule and the atoms, i.e. option 1 (equivalent to option 2 with the fragments being atoms).
The difference is so big when you talk about semiconductors because in the natural species, the dynamics is totally different from those including some impurities or defects.
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