Using the Bader analysis with the VASP code

One major issue with the charge density (CHGCAR) files from the VASP code is that they only contain the valance charge density. The Bader analysis assumes that charge density maxima are located at atomic centers (or at pseudoatoms). Aggressive pseudopotentials remove charge from atomic centers where it is both expensive to calculate and irrelevant for the important bonding properties of atoms.

Recently, the VASP developers have added a module (aedens) which allows for the core charge to be written out from PAW calculations. This module is included in the latest version of vasp (4.6.31 08Feb07). With this latest code, by adding the LAECHG=.TRUE. to the INCAR file, the core charge is written to AECCAR0 and the valance charge to AECCAR2. These two charge density files can be summed using the chgsum.pl script:

  chgsum.pl AECCAR0 AECCAR2

The bader analysis can then be done on this total charge density file:

  bader CHGCAR -ref CHGCAR_sum

One finally note is that you need a fine fft grid to accurately reproduce the correct total core charge. It is essential to do a few calculations, increasing NG(X,Y,Z)F until the total charge is correct.