[Wien] lapw1 runs too slow for H2 molecule

[Laurence Marks]

I think you have to use your second option, the same KMAX
(=RKMAX/min(RMT)) in all cases as well as the same RMT's

On Fri, Oct 10, 2008 at 10:49 AM, Stefaan Cottenier
<Stefaan.Cottenier at fys.kuleuven.be> wrote:
>
>> And as I said before, NEVER compare two calculations with different
>> RMTs/RKMAX against each other unless you normalize somehow to the same
>> reference.
>
> Some time ago I was pondering a situation where this was hard to
> avoid. It might be the right occasion to raise issue this here:
>
> It is about the calculation of the formation energy interstitial
> impurities (e.g. Al on the tetrahedral site in Si). To this end, one
> should calculate:
>
> * A=:ENE for a supercell with (for instance) 32 Si atoms and 1 Al atom
> * B=:ENE for a supercell with 32 Si atoms with the equilibrium volume
> per atom -- just bulk Si
> * C=:ENE for a supercell for fcc Al, with the equilibrium volume per
> atom and supercell volume roughly similar to the one of the Si supercell
>
> [The reason of taking supercells also for bulk Si and bulk Al is that
> the effective matrix size in :RKM changes abruptly with volume for
> small unit cells, and I don't dare to trust that by using simply the
> same RKM one has effectively the same matrix size as in the supercell
> for the impurity. But that's another issue, not the topic of this post.]
>
> Call C' the value of C divided by the number of atoms in the Al supercell.
>
> The formation energy for the tetrahedral Al impurity is then:
>
> B+C'-A
>
> So far, so good. In the case of Al and Si, one can take the same RMT's
> for both elements. And then one uses the same RKM for all three
> calculations, no problems appear. But what if the impurity is very
> different from Si, and it is needed to take a different RMT? For case
> A, the basis set size is determined by RKM/RMT_min (RMT_min is the
> smaller of the two RMT's). For case B, it is determined by RKM/RMT_Si,
> and for case C by RKM/RMT_imp : the effective basis set sizes will be
> different, is it then allowed to use these 3 energies in the same
> equation? I doubt so.
>
> One could adjust RKM, such that in all 3 cases the same Kmax is used.
> But that introduces variations in the energies that are of the same
> order of magnitude than the formation energies that are searched.
> Looks like a dangerous game.
>
> The solution I can imagine, is to use very large RKM values (e.g. 9).
> The dependence of :ENE on the basis set size will then be small, such
> that the effect described here does not matter very much.
>
> Has anybody another strategy?
> Or do I see problems where there actually aren't?
>
> Stefaan
>
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-- 
Laurence Marks
Department of Materials Science and Engineering
MSE Rm 2036 Cook Hall
2220 N Campus Drive
Northwestern University
Evanston, IL 60208, USA
Tel: (847) 491-3996 Fax: (847) 491-7820
email: L-marks at northwestern dot edu
Web: www.numis.northwestern.edu
Chair, Commission on Electron Crystallography of IUCR
www.numis.northwestern.edu/
Electron crystallography is the branch of science that uses electron
scattering to study the structure of matter.