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

[Anna Kuznetsova]

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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