[Wien] consistent RKmax and sphere size settings

[Peter Blaha]

Three comments:

RMT consistency is in particular important for total energies. If you 
compare the energy of 2 phases and their energy difference is just a few 
mRy, make sure you really have consistent RMTs and RKmax. Since the 
scaling with RKmax is not perfect, it might be wrong enough to lead to a 
wrong ground state (in particular when you are using a rather low RKmax 
value - with a fully converged RKmax the RMT dependency is reduced 
(except for possible core leakage !)

You mentioned core level shifts. This is something you simply have to 
test yourself. My expectation is, that it is NOT very crucial, as long 
as you have at least "similar" convergence. Calculate the core level 
shifts for one compound with large and small RMT and compare the 
results. If they are the same within acceptable errors, you can forget 
RMT problems for this property.
I'd expect that this should be general enough except when you have a 
very exotic compound.

lmax is uncritical, but for larger spheres lvns may have a larger 
influence as well as linearization errors (HDLOs for d or f electrons 
and large spheres), while GMAX may also be important for small spheres.

Am 08.04.2021 um 07:18 schrieb Pavel Ondračka:
> Thank you Laurence,
> 
> I was a bit worried because the FAQ you linked also says: "Of course
> you should use identical Mg+O spheres for MgO and Mg(OH)2 for
> consistency", so I was not 100% sure if keeping the same maximum K-
> vector Kmax is enough.
> 
> Should I also increase lmax and lvns for the larger spheres somehow? Or
> would you keep it the same for small and large N spheres?
> 
> Best regards
> Pavel
> 
> On Wed, 2021-04-07 at 15:33 -0500, Laurence Marks wrote:
>> Have a look at http://www.wien2k.at/reg_user/faq/rkmax.html. If (say)
>> with an RMT for the N of 1.6 a RKMAX of 6.5 is good enough, then when
>> you reduce the RMT to 1.3 you can reduce the RKMAX to 6.5*1.3/1.6 =
>> 5.28. This will not give you precisely the same relative convergence,
>> but is close.
>>
>> Another way is to say that an RKMAX of 7 is "OK" for RMTs of 2.0, an
>> RKMAX of 3 for RMTs of 0.5, then interpolate using a straight line.
>> This is similar.
>>
>> On Wed, Apr 7, 2021 at 3:24 PM Pavel Ondračka
>> <pavel.ondracka at email.cz> wrote:
>>> Dear Wien2k mailing list,
>>>
>>> I have a series of TiN and TiON amorphous-like structures where I
>>> have
>>> some large differences in spheres sizes for N atoms. In most of the
>>> structures the smallest N sphere is around 1.6-1.7, however in some
>>> I
>>> have few N atoms with 1.3 (the structures should be OK, this much
>>> smaller size is due to some rare local configuration which would
>>> correspond to something like N split interstitial in crystalline
>>> structure).
>>>
>>> My goal is to calculate core electron binding energies of N1s
>>> levels
>>> of
>>> many atoms in the structures (at least 200 core-hole calculations)
>>> and
>>> I need to be consistent over different structures in the series.
>>>
>>> So usually I would just check what is the smallest N sphere size in
>>> the
>>> whole set, and force it for all N atoms in all structures and than
>>> use
>>> the identical RKmax for all structures, just to be sure I'm
>>> consistent.
>>> This is unfortunatelly not very efficient with respect to the
>>> calculation speed as I have quite large cells (around 150 atoms).
>>> Is
>>> there another way how can I save some CPU time and keep the
>>> consistency?
>>>
>>> I was for example thinking if I can force somehow two different N
>>> sphere sizes (one for the N split intestitial, which I have usually
>>> just one in the whole cell and one larger for the rest of N atoms),
>>> than I would have consistent sphere size for the rest of N atoms in
>>> the
>>> series and I could change the RKmax to keep the same largest K-
>>> vector
>>> which should be enough to guarantee consistency for all N atoms
>>> expect
>>> the split interstitials (but I don't care that much about them).
>>> However as far as I understand this is not possible?
>>>
>>> Any ideas would be appreciated.
>>>
>>> Best regards
>>> Pavel Ondracka
>>>
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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