[Wien] should there be : one or more k points in kgen: organic material

[Peter Blaha]

> Once I have been advised from this mailing list to set just 1 k point in kgen when calculating an organic material. I believe it was because bands in an organic material were supposed to be narrow and E - not to change with k.  
> 
> I found some references [by Cornil, Bredas et.al] where the HOMO and LUMO bandwidths for a number of typical organic materials were reported as 0.2-1 eV broad. 
> 
> Taking into account such bandwidths, would there be still a fundamental reason for E not to change much with k? and would your advise still be to use just 1 k point in kgen?

When simulation a SINGLE molecule in a "large box", ALWAYS use 1 k-point.

For an organic SOLID there can of course be interaction (usually relative 
weak) between the molecules and one most likely needs more k-points.
This depends
a) on the unit cell (the larger the unit cell dimensions, the less 
k-points)
b) the desired accuracy (more k-points --> more accurate)

As for all other things: you MUST check your results for convergence. So 
start out with one k-point, save the results and (at least if possible) 
continue with a better mesh (and see if this gives any changes).
 

                                      P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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