[Pw_forum] Bader charges in bulk GaN

[Manjusha Chugh]

Hello

I am calculating bader charges from a cube file for hexagonal GaN bulk
system. The Ga and N layers in this compound are in the form of :
ABA'B'ABA'B'..... where the difference between A and A' layers is that they
are slightly shifted laterally with respect to each other. Similarly, the B
and B' layers. First, I have generated a charge density cube file by doing
post-processing of the wavefunctions collected from the scf calculation of
bulk 2x2x2 GaN system in Quantum Espresso. Then I ran the bader code for
the cube file to get the charges. The k-point grid I used is 12x12x12 .

The total charge of the system comes out to be correct (the expected
valence charge of the system). Also, the charges on atoms of the same layer
are same. But the problem is that the charges on Ga atoms of layers A and
A' (and also charges on N atoms of layers B and B') are not same. This is a
bulk system and all the Ga atoms are in same chemical environment. They are
tetrahedrally bonded to 4 N atoms (and same is true for N atoms, N atoms in
layers B and B' are tetrahedrally bonded to 4 Ga atoms). So, I expected
that the charges on all the Ga atoms in this bulk system should be same.
Can someone please tell me what I am missing?
The z-valence in the pseudopotential file for Ga and N are 13 and 5
respectively.

Here is the content from ACF.dat file :

#    X            Y               Z           CHARGE   MIN DIST   ATOMIC VOL
--------------------------------------------------------------------------------
1   18.2640    10.5444    19.7982     11.4266      1.5237      66.6395
2    15.2200   5.2722       19.7982     11.4265      1.5237      66.6358
3    12.1760   10.5444     19.7982     11.4266      1.5237      66.6395
4    9.1320    5.2722        19.7982     11.4266      1.5237     66.6395
5    18.2640   10.5444      3.7320        6.5809      1.7181     92.3652
6   15.2200   5.2722        3.7320        6.5809      1.7181     92.3652
7   12.1760   10.5444      3.7320        6.5809      1.7181     92.3652
8   9.1320     5.2722       3.7320        6.5809       1.7181     92.3652
9   3.0428     1.7567       4.9495       11.3747      1.5089     64.9901
10   9.1308    1.7567      4.9495       11.3747      1.5089     64.9901
11   6.0868    7.0289       4.9495      11.3747      1.5088     64.9901
12   12.1748   7.0289      4.9495      11.3747      1.5089     64.9901
13   3.0428    1.7567       8.6815       6.6179      1.7392     93.7406
14   9.1308    1.7567       8.6815       6.6179      1.7393     93.7406
15   6.0868    7.0289       8.6815       6.6179      1.7392     93.7461
16   12.1748    7.0289     8.6815       6.6179      1.7392     93.7461
17   18.2640   10.5444    9.8991      11.4265      1.5237     66.6339
18   15.2200   5.2722      9.8991      11.4265      1.5237     66.6358
19   12.1760   10.5444     9.8991     11.4265      1.5237     66.6339
20   9.1320     5.2722      9.8991      11.4265      1.5237     66.6358
21   18.2640    10.5444    13.6310      6.5809     1.7181     92.3652
22   15.2200    5.2722     13.6310       6.5809     1.7181     92.3652
23   12.1760    10.5444    13.6310      6.5809      1.7181     92.3652
24   9.1320     5.2722      13.6310       6.5809      1.7181      92.3652
25   3.0428    1.7567       14.8486      11.3747      1.5089     64.9901
26   9.1308    1.7567       14.8486      11.3747      1.5089      64.9901
27   6.0868    7.0289       14.8486      11.3747      1.5088      64.9901
28   12.1748   7.0289      14.8486       11.3747     1.5089      64.9901
29   3.0428    1.7567       18.5806        6.6179      1.7392     93.7424
30   9.1308    1.7567       18.5806        6.6178      1.7393     93.7388
31   6.0868    7.0289       18.5806        6.6178      1.7392     93.7388
32   12.1748   7.0289       18.5806       6.6178      1.7392     93.7388
--------------------------------------------------------------------------------
VACUUM CHARGE: 0.0000
VACUUM VOLUME: 0.0000
NUMBER OF ELECTRONS: 288.0005

I expected that the charges on Ga atoms having numbers 1,2,3,4 ; 9,10,11,12
; 17,18,19,20 ; 25,26,27,28 should all be same. Same should be true for N
atoms.


Thank you in advance.
Manjusha
IIT Kanpur, India
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