[Wien] NON-ZERO forces for zinc-blend GaAs

[Oleg Rubel]

Dear Wien2k Users,

I met a problem with NON-ZERO forces while performing SCF calculation for
a zinc-blend GaAs supercell.


Structure: My supercell belongs to a rhombohedral crystal system with 4
non-equivalent atoms and the edges along [-1 1 0], [101], and [111]
directions (please see the structure file below). (I need this structure
for further calculation of {111} antiphase boundaries.) This structure
cannot be reduced down to a primitive 2-atom basis due to explicit
specifying of Ga and As atoms as non-equivalent (As1, As2, Ga1, Ga2).


Problem: When I perform 'run_lapw -ec 0.00005', I do see non-zero forces,
though I expect the opposite since the structure is perfect (I verified
that by measuring of interatomic distances and angles in XCRYSDEN):

marc-hn:~/wien_work/GaAs111APB/4atoms/noAPB> grep :FOR *.scf
:FOR001:   1.ATOM      0.699      0.000      0.000      0.699
:FOR002:   2.ATOM      0.699      0.000      0.000      0.699
:FOR003:   3.ATOM      0.647      0.000      0.000     -0.647
:FOR004:   4.ATOM      0.647      0.000      0.000     -0.647
:FOR001:   1.ATOM      0.603      0.000      0.000      0.603
:FOR002:   2.ATOM      0.603      0.000      0.000      0.603
:FOR003:   3.ATOM      0.560      0.000      0.000     -0.560
:FOR004:   4.ATOM      0.560      0.000      0.000     -0.560
:FOR001:   1.ATOM      0.021      0.000      0.000      0.021
:FOR002:   2.ATOM      0.021      0.000      0.000      0.021
:FOR003:   3.ATOM      0.013      0.000      0.000     -0.013
:FOR004:   4.ATOM      0.013      0.000      0.000     -0.013

I also met the similar problem while calculation elastic constants of
zinc-blend GaAs using ELAST. It also creates a rhombohedral structure with
a 2-atom basis and also gives non-zero forces.

There was NO warning messages neither during initialization nor during SCF
runs. I also use well approver Rmt radiuses (2.0 for both atoms), safe RKM
= 7, and dense k-point mesh (total 1000 k-points in the BZ).  Everything
runs smoothly and the total energy is converged within several SCF cycles.
I do not understand what is wrong?

In addition, I verified the structure by setting Ga and As atoms as
equivalent. Then wien2k reduces the structure automatically to a 2-atom
FCC basis as it should be for the zinc-blend structure, and then
everithing is fine: NO forces!


Any suggestions and/or comments are highly welcome.

Thank you in advance,

Yours sincerely,

Oleg Rubel

===========================
Faculty of Physics
Philipps University Marburg
Renthof 5, 35032 Marburg, Germany
E-mail: Oleg.Rubel at physik.uni-marburg.de
Homepage: http://www.staff.uni-marburg.de/~rubel/


marc-hn:~/wien_work/GaAs111APB/4atoms/noAPB> cat *.struct
GaAs111APB
R   LATTICE,NONEQUIV.ATOMS:  4 160 R3m
MODE OF CALC=RELA unit=bohr
  7.502403  7.502403 36.754118 90.000000 90.000000120.000000
ATOM  -1: X=0.00000000 Y=0.00000000 Z=0.00000000
          MULT= 1          ISPLIT= 4
As1        NPT=  781  R0=0.00005000 RMT=    2.0000   Z: 33.0
LOCAL ROT MATRIX:    1.0000000 0.0000000 0.0000000
                     0.0000000 1.0000000 0.0000000
                     0.0000000 0.0000000 1.0000000
ATOM  -2: X=0.50000000 Y=0.50000000 Z=0.50000000
          MULT= 1          ISPLIT= 4
As2        NPT=  781  R0=0.00005000 RMT=    2.0000   Z: 33.0
LOCAL ROT MATRIX:    1.0000000 0.0000000 0.0000000
                     0.0000000 1.0000000 0.0000000
                     0.0000000 0.0000000 1.0000000
ATOM  -3: X=0.12500000 Y=0.12500000 Z=0.12500000
          MULT= 1          ISPLIT= 4
Ga1        NPT=  781  R0=0.00005000 RMT=    2.0000   Z: 31.0
LOCAL ROT MATRIX:    1.0000000 0.0000000 0.0000000
                     0.0000000 1.0000000 0.0000000
                     0.0000000 0.0000000 1.0000000
ATOM  -4: X=0.62500000 Y=0.62500000 Z=0.62500000
          MULT= 1          ISPLIT= 4
Ga2        NPT=  781  R0=0.00005000 RMT=    2.0000   Z: 31.0
LOCAL ROT MATRIX:    1.0000000 0.0000000 0.0000000
                     0.0000000 1.0000000 0.0000000
                     0.0000000 0.0000000 1.0000000
   6      NUMBER OF SYMMETRY OPERATIONS
 1 0 0 0.0000000
 0 1 0 0.0000000
 0 0 1 0.0000000
       1
 0 0 1 0.0000000
 1 0 0 0.0000000
 0 1 0 0.0000000
       2
 0 1 0 0.0000000
 0 0 1 0.0000000
 1 0 0 0.0000000
       3
 0 1 0 0.0000000
 1 0 0 0.0000000
 0 0 1 0.0000000
       4
 1 0 0 0.0000000
 0 0 1 0.0000000
 0 1 0 0.0000000
       5
 0 0 1 0.0000000
 0 1 0 0.0000000
 1 0 0 0.0000000
       6