[Pw_forum] force convergence in vc-relax

[Paolo Giannozzi]

On Mon, 2015-04-13 at 00:16 -0700, Florian Altvater wrote:

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

The explanation is available in the part of code that perform the
convergence check on forces

P.
> 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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> 
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-- 
 Paolo Giannozzi, Dept. Chemistry&Physics&Environment, 
 Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
 Phone +39-0432-558216, fax +39-0432-558222