# [Pw_forum] force convergence in vc-relax

Florian Altvater altvater at berkeley.edu
Mon Apr 13 09:16:22 CEST 2015

```Hi,
did this issue ever get resolved/answered? I would also be very
interested in a solution/explanation of the behavior.

I also get relaxations where individual forces are higher than the
threshold, without the line stating that "SCF correction compared to
forces is large", in case that would be a reason here. Also happens in
regular "relax" calculations.

Thanks so much,
Florian

--
Florian Brown-Altvater
PhD Candidate
Neaton group
UC Berkeley/LBNL

Am 08.04.2015 um 12:07 schrieb 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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