[Pw_forum] residual forces along z-dimension after vc-relax

[Giuseppe Mattioli]

Dear Anna
If I remember well, the nearest-neighbor distance along z in the  
wurtzite crystals is not an inherent characteristic of its hexagonal  
symmetry, and should be therefore optimized also in a "relax"  
calculation. I suppose that when you shift from the hexagonal to the  
orthorombic supercell there are symmetry differences in the stress  
tensor that lead to differences (hopefully small, if the calculations  
are well-converged) in the c lattice parameters.
HTH
Giuseppe

Giuseppe Mattioli
ISM-CNR
Italy

P.S.
>   ecutwfc     = 80,
>   ecutrho     = 700,
probably too much for paw pseudopotentials. Didi yo find some  
instability of results at lower cutoffs?

Quoting Anna Kuznetsova <kuznetsova.ann at gmail.com>:

> Dear users,
>
> I am trying to achieve the perfect  relaxation of hexagonal unit cell in
> orthorhombic coordinates, however, I still see quite substantial residual
> forces along z-dimension. Doing finer k-mesh sampling does not help with
> the issue.
> I am quite sure that the coordinates of atoms in each layer are correct, so
> I was expecting that vc-relax will result only in the change in the unit
> cell dimensions, but not the changes of atomic positions  inside it.
>
> Please, give me some hints of what could be wrong in my input to help me
> achieve full relaxation of the unit cell.
>
>
> Input (also attached):
>
> &CONTROL
>   calculation = "vc-relax",
>   pseudo_dir  = "/u/annakuz/espresso/pseudo",
>   outdir      = "/lustre/work1/annakuz/tmp4",
> /
> &SYSTEM
>   ibrav       = 8,
>   celldm(1)   = 5.781869696282005, celldm(2)  = 1.732050807568878,
> celldm(3) = 1.632993161855452,
>   nat         = 8,
>   ntyp        = 2,
>   ecutwfc     = 80,
>   ecutrho     = 700,
>   occupations='smearing', smearing='gauss', degauss=0.03
> /
> &ELECTRONS
>   conv_thr    = 1.D-12
> /
> &IONS
> /
> &CELL
> /
> ATOMIC_SPECIES
> Si  28.0855 Si.pz-n-kjpaw_psl.0.1.UPF
> C   12.0107  C.pz-n-kjpaw_psl.0.1.UPF
> ATOMIC_POSITIONS {crystal}
> Si   0.00  0.00  0.00
> C    0.00  0.00  0.375
> Si   0.50  0.50  0.00
> C    0.50  0.50  0.375
> Si   0.50  0.166666667  0.5
> C    0.50  0.166666667  0.875
> Si   1.00  0.666666667  0.5
> C    1.00  0.666666667  0.875
>
> K_POINTS automatic
> 15 8 9 1 1 1
>
>
> Output is attached but here is the final result of relaxation showing
> forces still present in z-direction:
>
> ............
>      convergence has been achieved in  31 iterations
>      Forces acting on atoms (Ry/au):
>      atom    1 type  1   force =     0.00000000   -0.00000037    0.00011491
>      atom    2 type  2   force =     0.00000000    0.00000026   -0.00011491
>      atom    3 type  1   force =     0.00000000   -0.00000037    0.00011493
>      atom    4 type  2   force =     0.00000000    0.00000030   -0.00011491
>      atom    5 type  1   force =     0.00000000    0.00000037    0.00011498
>      atom    6 type  2   force =     0.00000000   -0.00000029   -0.00011498
>      atom    7 type  1   force =     0.00000000    0.00000036    0.00011495
>      atom    8 type  2   force =     0.00000000   -0.00000027   -0.00011497
>      Total force =     0.000325     Total SCF correction =     0.000000
>
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=
> -0.09
>   -0.00000088   0.00000000   0.00000000         -0.13      0.00      0.00
>    0.00000000  -0.00000088   0.00000000          0.00     -0.13      0.00
>    0.00000000   0.00000000   0.00000001          0.00      0.00      0.00
>
>      bfgs converged in   3 scf cycles and   2 bfgs steps
>      (criteria: energy < 0.10E-03, force < 0.10E-02, cell < 0.50E+00)
>      End of BFGS Geometry Optimization
>      Final enthalpy =    -250.0275444323 Ry
> Begin final coordinates
>      new unit-cell volume =    547.64431 a.u.^3 (    81.15251 Ang^3 )
>
> CELL_PARAMETERS (alat=  5.78186970)
>    0.998918439   0.000000000   0.000000000
>    0.000000000   1.730177232   0.000000000
>    0.000000000   0.000000000   1.639355189
> ATOMIC_POSITIONS (crystal)
> Si       0.000000000  -0.000000091  -0.000421399
> C        0.000000000  -0.000000003   0.375421398
> Si       0.500000000   0.499999909  -0.000421402
> C        0.500000000   0.499999991   0.375421397
> Si       0.500000000   0.166666758   0.499578593
> C        0.500000000   0.166666675   0.875421409
> Si       1.000000000   0.666666758   0.499578597
> C        1.000000000   0.666666671   0.875421408
> End final coordinates
>
>     Thank you for help!
> --
> Best regards,
> Anna


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    Giuseppe Mattioli
    CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
    v. Salaria Km 29,300 - C.P. 10
    I 00015 - Monterotondo Stazione (RM)
    Tel + 39 06 90672836 - Fax +39 06 90672316
    E-mail: <giuseppe.mattioli at ism.cnr.it>