[Pw_forum] force convergence in vc-relax

[Tianli Feng]

Dear All,

I would really appreciate if anyone can help to explain a output in
relaxing a structure. I used vc-relax to relax a lattice with forc_conv_thr
set as 1.0D-9. After 44 steps, the results converge as following:

     Forces acting on atoms (Ry/au):

     atom    1 type  1   force =     0.00000638    0.00000000    0.00000092
     atom    2 type  1   force =     0.00000602    0.00000000    0.00000671
     atom    3 type  1   force =    -0.00000074    0.00000000    0.00000243
     atom    4 type  1   force =     0.00000358    0.00000000   -0.00000082
     atom    5 type  2   force =    -0.00000821    0.00000000   -0.00000732
     atom    6 type  2   force =    -0.00000635    0.00000000   -0.00000202
     atom    7 type  2   force =     0.00000337    0.00000000   -0.00000058
     atom    8 type  2   force =    -0.00000406    0.00000000    0.00000068

     Total force =     0.000018     Total SCF correction =     0.000002
     SCF correction compared to forces is large: reduce conv_thr to get
better values

     bfgs converged in  44 scf cycles and  43 bfgs steps
     (criteria: energy <  1.0E-04, force <  1.0E-09, cell <  5.0E-01)

The last line says the relaxation converges with force <1.0E-9, but I saw
the forces acting on each atom are not smaller than 1e-9. Does anyone has
experience on relaxing a lattice and know what is happening? Many thanks!!

Sincerely,
Best wishes,
Tianli
------------
Tianli Feng
Mechanical Engineering, Purdue University
---The truth is not what you see, but what it is.
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