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

[Stefaan Cottenier]

>>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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