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

[Pascal Boulet]

Hello,

With these small residual forces I would say that your structure is well converged.

Pascal

Le 16 janv. 2015 à 19:42, Anna Kuznetsova <kuznetsova.ann at gmail.com> a écrit :

> 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
> <2h_sic_pz_vcrelax5.rx.out><2h_sic_pz_vcrelax5.rx.in>_______________________________________________
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