[Wien] NON-ZERO forces for zinc-blend GaAs

[Oleg Rubel]

Dear Stefaan,
dear Laurence,

thank you very much for the suggestions.


Dear Ulrich,

> I didn't look at your cell carefully, but I think in your case symmetry
> does not force all bonds being of equal length.

All bonds seems to be of the equal length (please see the message
"Re: [Wien] NON-ZERO forces for zinc-blend GaAs / *.outputnn"), but thank
you anyway.


Yours,

Oleg Rubel


On Mon, 12 Mar 2007, Stefaan Cottenier wrote:

>
> >>But once more, a force of 0.0x is zero in practice.
> >>
> >>
> >
> >Oh yes, but for orthorhombic (primitive) structures forces are exactly
> >zero for ALL SCF iterations.
> >
> Determine the spacegroup and Wyckoff positions (case.outputsgroup) of
> your (super)cell and compare to crystallographic tables: in the case
> where you find forces that are exactly zero (I guess you mean here: not
> printed), there will not be free positions in the coordinates.
>
> >Interesting: the first SCF cycle right after
> >dstart does give zero forces!?
> >
> >
> Pretty meaningless, I would say.
>
> >>Also, -ec 0.00005 is not a very high convergence; -ec 0.00001 is
> >>better and you should also use -cc
> >>
> >>
> >
> >What would a "good" limit for -cc? May I set several convergence criteria
> >simultaneously, e.g. 'run_lapw -ec 0.00001 -fc 0.001 -cc ?????' ?
> >
> >
> >
> Yes you can. This is another issue where a small modification of the
> code could be useful, hence:
>
> AUTHORS: If  "-cc 0.00001 -fc 0.05" is used, and it happens that force
> convergence is reached earlier than charge convergence, then all
> iterations after reaching force convergence use FOR instead of TOT. For
> big cases, this consumes a lot of extra computing time. It would be
> helpful if when specifying -fc plus another convergence criterium, FOR
> is used only after satisfying *all* convergence criteria.
>
> >I experienced problem with convergency when I go up to 20-40 atoms per
> >supercell: the more atoms the more difficult to converge. But, for some
> >reasons, it does not happen for 'P'-lattice. Is it a general property of
> >'R'-lattices?
> >
> >
> >
> Probably not, but hard to comment on in general (I refrain from comments
> on convergence issues anyway, as other people in this list know much
> better what they are talking about ;-) )
>
> Stefaan
>
>
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