[Pw_forum] Ionic relaxation with BFGS

[feng.zimin at ireq.ca]

Dear friends,

I have a quick question about ionic relaxation with BFGS. I tried
searching for it in the archive but didn't find anything useful.

When the relaxation ends, it doesn't seem to me that the force is below
the value of **forc_conv_thr**. Actually AFTER the final step, it does
yet another calculation of the forces (with the same atomic positions as
the previous step), and this time the value of the total force is
usually much bigger than the previous step.

So why does it stop before the forces are below the threshold and why
does it do an extra step producing a much bigger force value?  (some
remarks on the choice of the parameters: I kept bringing down those
threshold values but it doesn't seem to me they are making any
difference...)

Thank you very much in advance for every single bit of your response!

Sincerely,

Feng, Zimin
Physics, McGill University

The full input/output files are attched and here is an excerpt from
them:

================= IN =================
&control
    etot_conv_thr=1.0D-15,
    forc_conv_thr=1.0D-11,
    nstep=900
 &electrons
    conv_thr=1.0D-14,
    mixing_beta=0.7
 /
&ions
    ion_dynamics='bfgs',
    upscale=5000,
/
&cell
    cell_dynamics='bfgs',
    cell_factor=2.5,
    press_conv_thr=0.05


================== OUT ======================

(....second to last step ...)

     Forces acting on atoms (Ry/au):

     atom    1 type  1   force =     0.00000000    0.00000000
0.00000000
     atom    2 type  1   force =    -0.00000000   -0.00000000
0.00000000
     atom    3 type  1   force =    -0.00000000    0.00000000
0.00000000
     atom    4 type  1   force =     0.00000000   -0.00000000
-0.00000000
     atom    5 type  2   force =    -0.00000000    0.00000000
0.00000274
     atom    6 type  2   force =    -0.00000000   -0.00000000
-0.00000274
     atom    7 type  2   force =     0.00000000   -0.00000000
-0.00000274
     atom    8 type  2   force =     0.00000000    0.00000000
0.00000274

     Total force =     0.000005     Total SCF correction =     0.000000


     entering subroutine stress ...

          total   stress  (Ry/bohr**3)                   (kbar)     P=
0.00
   0.00000000   0.00000000  -0.00000000          0.00      0.00
-0.00
   0.00000000   0.00000000  -0.00000000          0.00      0.00
-0.00
   0.00000000  -0.00000000   0.00000003          0.00     -0.00
0.00


     The maximum number of steps has been reached.

     End of BFGS Geometry Optimization
Begin final coordinates
     new unit-cell volume =    389.25790 a.u.^3 (    57.68207 Ang^3 )

.....(omited)......

     Forces acting on atoms (Ry/au):

     atom    1 type  1   force =     0.00000000    0.00000000
0.00000000
     atom    2 type  1   force =     0.00000000    0.00000000
-0.00000000
     atom    3 type  1   force =     0.00000000    0.00000000
0.00000000
     atom    4 type  1   force =     0.00000000    0.00000000
0.00000000
     atom    5 type  2   force =     0.00000000   -0.00000000
0.00155468
     atom    6 type  2   force =     0.00000000   -0.00000000
-0.00155468
     atom    7 type  2   force =    -0.00000000    0.00000000
-0.00155468
     atom    8 type  2   force =     0.00000000   -0.00000000
0.00155468

     Total force =     0.003109     Total SCF correction =     0.000000


     entering subroutine stress ...

          total   stress  (Ry/bohr**3)                   (kbar)     P=
-61.77
  -0.00045413   0.00000000   0.00000000        -66.80      0.00
0.00
   0.00000000  -0.00045413   0.00000000          0.00    -66.80
0.00
   0.00000000   0.00000000  -0.00035143          0.00      0.00
-51.70
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