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

[Stefaan Cottenier]

Did you either use -fc as convergence criterium or did changed TOT to 
FOR manually for the last iteration(s)? If not, then you did not 
calculate the Pulay corrections. Having them as well, should cancel the 
values you have now back to zero.

Note there is a different treatment for positions that cannot have a 
force by symmetry (no :FOR lines are printed at all) and positions that 
can have a force -- which might be 'accidentally' zero (:FOR is printed, 
but you need the FOR keyword to have the complete force here).

AUTHORS : as far as I know, there is no straightforward way to recognize 
afterwards (in files of a saved calculation for instance, which do not 
necessarily include case.in2) whether or not FOR was used. A simple 
remedy would be to print a line before the :FOR/:FGL lines: "Calculated 
with TOT/FOR in case.in2 : (NO) Pulay corrections added"

Stefaan


Oleg Rubel wrote:

>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
>
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