[Pw_forum] relax question

[Xunlei Ding]

Hi,

In the vasp forum, one set the parameter like this:
EDIFF: 1E-07
EDIFFG: -0.0005
 
And in the reply it is said, "Anyways, your ediffg is very low. A strict force
criteria is -0.001, and many people would use -0.01 or higher. If you look at
the change in the energy as you go from a force of 0.01 eV/Ang to 0.001 eV/Ang,
it is usually negligible."

You can check the difference of results when you set force_conv_thr as 1.d-3 and
1.d-4. If the difference is negligible, why use too strict threshold?

Yours,
Ding Xunlei


Quoting xu yuehua <njuxuyuehua at gmail.com>:

> about why i use etot_conv_thr
> as well as  force_conv_thr= 1.d-6 Ry
> as usual,in vasp and castep software,about 1.d-4 ev is ok
> why it is so different between these DFT software ?
> i don not understand.
> need your help
> 
> 
> 
> 2007/7/13, Stefano de Gironcoli <degironc at sissa.it>:
> >
> > a tipical bond stiffness, if I recall correctly, is in the range of
> > 0.1-1.0
> > Ry/bohr^2 thus a threshold of 1.d-6 Ry/bohr in the force means that you
> > are
> > asking a numerical accuracy in your interatomic distances of the order of
> > 1.d-5-1.d-6 bohr... are you sure you really need it ?
> > best regards,
> > stefano deGironcoli, SISSA & DEMOCRITOS
> >
> > Quoting xu yuehua <njuxuyuehua at gmail.com>:
> >
> > > hello all
> > > i am doing relax.
> > > in my inputfile,
> > > i use Bohr> and the output is
> > >    atom   1 type  1   force =     0.00000290    0.00000095   -0.00000364
> > >      atom   2 type  1   force =     0.00000081   -0.00000108
> > 0.00000033
> > >      atom   3 type  1   force =    -0.00000435   -0.00000425   -
> > 0.00000160
> > >      atom   4 type  1   force =    - 0.00000312   -0.00000321   -
> > 0.00000080
> > >      atom   5 type  1   force =    -0.00000446   -0.00000036   -
> > 0.00000273
> > >      atom   6 type  1   force =    -0.00000048    0.00000230
> > 0.00000112
> > >      atom   7 type  1   force =     0.00000164    0.00000118
> > 0.00000060
> > >      atom   8 type  1   force =     0.00000131   -0.00000040
> > 0.00000145
> > >      atom   9 type  2   force =    -0.00000362    0.00000154   -
> > 0.00000201
> > >      atom  10 type  2   force =    - 0.00000029    0.00000191
> > 0.00000302
> > >      atom  11 type  2   force =     0.00000576   -0.00000134
> > 0.00000194
> > >      atom  12 type  2   force =     0.00000390    0.00000276
> > 0.00000233
> > >
> > > Total force =     0.000015     Total SCF correction =     0.000054
> > >      SCF correction compared to forces is too large, reduce conv_thr
> > >
> > >      The maximum number of steps has been reached.
> > > my qustion is ,force at every atom is ok ,but why  the program asks to
> > > reduce conv_thr
> > > my conv_thr is not enough small ?
> > > or my total relax step (50) is not adequate?
> > > is there any principle about these parameter?
> > > need your help
> > > thanks a lot
> > > --
> > > Xu Yuehua
> > > physics Department of Nanjing university
> > > China
> > >
> >
> >
> >
> >
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> 
> 
> 
> -- 
> Xu Yuehua
> physics Department of Nanjing university
> China
> 




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