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I want to analys the changes charge between two atoms, so how to use VASP to calculate the occupation number of the electrons. Thank you very much.

You could integrate (numerically) the LDOS of the seperate atoms. Make sure that the chosen Wigner radii do not overlap.

Note however that this will not give you absolute values on the occupation.

If you are more interested in the charge distribution, then calculate you full system (CHGCAR_f), and then calculate with everything fixed your 2 subsystems (CHGCAR_1 and CHGCAR_2)
from this you can then get the charge transfer by:
CHGCAR_f - CHGCAR_1 - CHGCAR_2

Danny

[quote="joannase"]I want to analys the changes charge between two atoms, so how to use VASP to calculate the occupation number of the electrons. Thank you very much.

[/quote]

The atomic charges reported by VASP at the end of the OUTCAR file correspond to the total charge in the Wigner-Seitz cell of the atoms. The sum of these charges does not give in general the total valence charge of the system since the Wigner-Seitz cell is just an approximation to the individual atomic region. It is therefore very difficult to infer the charge transfer from these values. To get a better estimation of the total charge per atom, you can use the Bader program by Henkelman et al (see the thread "Bader charge" in the section "Important threads" of the forum "Physics Questions" of this site). By substracting the total charge in the Bader region of each atom to the atomic valence charge you can get an estimation of the charge transfer.

- jlrch