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Is it possible to obtain ion's charge state in VASP?
If it is, then how to do that?

Thanks in advance

how do you define "charge" ? all definitions of defining charge (e.g. Mullikan) are arbitrary.
So, what you can do is to define your RWIGS and check the charge allocated within the sphere you have defined. But be careful when interpreting what you see (all depends on your choice of RWIGS, complicated if many sorts of atoms are there, etc) -- also check the manual for this.
But no matter what you do, you'll have a hard time to get oxygen as (-2) in any oxide ....
Overall, you are better of with "changes" of charges.

how do you define "charge" ? all definitions of defining charge (e.g. Mullikan) are arbitrary.
So, what you can do is to define your RWIGS and check the charge allocated within the sphere you have defined. But be careful when interpreting what you see (all depends on your choice of RWIGS, complicated if many sorts of atoms are there, etc) -- also check the manual for this.
But no matter what you do, you'll have a hard time to get oxygen as (-2) in any oxide ....
Overall, you are better of with "changes" of charges.

"how do you define "charge" ?"

so, actually, i'm didn't think about it. because my question is related to the state of ion, in which it has an additional or missing electrons in comparison with free atom. as far as i know this state is commonly refered to as "charge state".

"So, what you can do is to define your RWIGS and check the charge allocated within the sphere you have defined. But be careful when interpreting what you see (all depends on your choice of RWIGS, complicated if many sorts of atoms are there, etc) -- also check the manual for this."

Well, i'll try to do this way. the point is that i thought of, so to say, more "direct' way of charge state determinition.

http://link.aip.org/link/?APPLAB/84/2289/1

In this paper peoples report the charge states obtained by VASP. So maybe they did it the way you described

Thanks

I guess you mean "charge state" is how many charge partition into ion right?
Recently,I usually use the "bader partition method"
I think it's a very powerful tool satisfy what you need

http://theory.cm.utexas.edu/bader/

So what about a system that is not charge neutral, like a complex with charge of -2, for instance. Am I to assume (as I understand it) that Mullikan charge is arbitrary (not defined) so I don't have to worry about charge in my simulation? Or do I need to tie off the charge of the complex with a +2 ion to ensure the system has charge neutrality prior to the simulation. I would think adding an ion would change the physical results of the simulation, as the structure for the POSCAR would have different symmetry and also another element to treat. Please someone confirm that I am correct in just ignoring the charge in this case! ~Thanks