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

[Oleg Rubel]

Thank you Stefaan,
thank you Laurence,


> 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. Interesting: the first SCF cycle right after
dstart does give zero forces!?

> 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 ?????' ?

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?


Thank you once again.

Yours,

Oleg Rubel


On Mon, 12 Mar 2007, Stefaan Cottenier wrote:

>
> >I exchanged TOT by FOR in all *.in2* files
> >and run 'run_lapw -ec 0.00005' again. Here is the result:
> >
> >marc-hn:~/wien_work/GaAs111APB/4atoms/noAPB> grep :FOR *.scf
> >:FOR001:   1.ATOM      0.072      0.000      0.000     -0.072
> >
> >The problem is not gone
> >
> >
> Only now I notice that you did use -fc, as in your previous post you see
> the value for e.g. :FOR001 changing in the last iteration (thought all
> these lines were for one structure file, sorry):
>
> >:FOR001:   1.ATOM      0.699      0.000      0.000      0.699
> >
> >:FOR001:   1.ATOM      0.603      0.000      0.000      0.603
> >
> >:FOR001:   1.ATOM      0.021      0.000      0.000      0.021
> >
> Two things about that:
>
> 1) Such a small force (this is mRy/au, not meV/Ang !) is essentially zero.
>
> 2) The difference betwee the final-minus-1 and final-minus-2 iterations
> is at the 0.1 level. That means that alos your final force is converged
> only upon that level (your new 0.072 value when you add one iteration
> confirms that). At that level of convergence, even the exact 0.000 lies
> within the numerical spread.
>
> 3) If you converge further, you will probably still find a force at the
> 0.0x level (or perhaps a bit smaller). Increasing basis set size (up to
> even 9.0) would be needed to bring that one closer to zero. But once
> more, a force of 0.0x is zero in practice.
>
> Stefaan
>
>
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