[Wien] calculation quality

[Pablo de la Mora]

Thank you
    Pablo

Peter Blaha escribió:
> No, the atomic number is not really a good parameter.
> The needed value of RKmax depends much more on the type of
> valence electrons and an "old" rule said:
> for s,p wavefunctions you need RKmax 6-7
>      d                                7-8
>      f                                8-9
>
> So a "heavier" element, which has only s,p electrons (eg. S, Se, Te) needs
> a much samller RKmax than a "light" Fe atom.
>
> To some extend we try to "correct" this automatically in a compound and the
> setup make a sphere of sp-atoms smaller than the sphere of 3d or even 4f elements.
>
> Of course, within a row in the periodic table, your arguments would be correct,
> i.e. a O or F atom needs a larger RKmax than Be or B. Thus for a CO molecule
> I'd make the C sphere smaller than the O-sphere (this is NOT done yet by setrmt).
>
> H, which has only s-electrons, is even more special and a much smaller RKmax is enough.
>
> k-points: sure, the size of the unit cell matters and for one and the same material
> the k-mesh scales with volume (eg. when creating a supercell).
> But other properties may be even more important. For certain magnetic metallic
> materials you may need 100-10000 times more k-points than for an insulator,
> although the volume might be identical.
>
> There is NO other choice than: start with a realistic (but small) k-mesh and test
> the result of the desired properties with respect to k-mesh.
>
> What "realistic, but small" means, needs a bit experience, but there were many
> discussions about that in the mailing-list or in WIEN2k-workshops.
>
> Pablo de la Mora schrieb:
>   
>> Dear Wien-users,
>> There are two parameters that meassure the quality of a calculation; RxK
>> and the number of k-points.
>>
>> With respect to RxK; for hydrogen R is quite small but it also has one 
>> nuclear charge, so a small RxK parameter is acceptable.
>> So my question is: would a more complex parameter be more meaningful?
>> Such as
>>         RxK/n
>> where n is the atomic number?
>> The idea behind is that the nuclear potential increases with n, so a
>> larger R is needed and R/n would be more or less constant.
>>
>> With respecto to the number of k-points (Nk) with the size of the cell 
>> the number of k-points should be reduced to maintain the same quality, 
>> so this parameter could be modified to VxNk.
>>
>>        Pablo de la Mora
>>
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>
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