[Wien] Electron density at the nucleus (Electron capture nuclear decay rate work)

[Peter Blaha]

I'd have to recheck how the Fe-Isomershift core contributions change under
pressure, but the longer I think about the problem, the more I understand
that the Be-1s density gets more delocalized under compression.

If the neighbors are far away, the Be 1s orbital sees for long time a kind of
Z/r potential and only at rather large distances the potential bends over
and gets attractive again because there is a neighboring atom.
If you compress, the potential gets attractive at smaller distances, i.e.
the localized 1s orbital will expand a bit, because it sees a more attractive
potential at its tail.
Also the contraction of the 2s orbital drives the 1s electron into expansion:
More 2s charge in the core region "weakens" the attractive potential for the
1s orbital.

Certainly, GGA-DFT potentials have their shortcuts and we know that the
core states are not localized enough, but I believe qualitatively the
behavior is correct.
For sure I believe, that WIEN2k is one of the very few programs which can deal
with such problems because of the numerical all electron basis set. Of course
if DFT is the problem,.....

Pavel Novak schrieb:
> let me comment. I do not recommend to use the Lundin-Eriksson 
> functional. While the contact hyperfine field for 3d atoms is improved, 
> we realized that it violates important sum rule for the 
> exchange-correlation hole, which is imposed by the density functional 
> theory. This brings several shortcomings e.g. incorrect energy of the 
> core states. I doubt that any local or semilocal Vxc can provide 
> reliable value of the core density at the nucleus. For bcc Fe Akai and 
> Kotani (Hyperfine Interactions, 120/121, 3, 1999 obtained good contact 
> field using the optimised effective potential method, but this is 
> computationally expensive.
> Regards
> Pavel Novak
> 
> On Wed, 21 Apr 2010, Laurence Marks wrote:
> 
>> A few comments, and perhaps a clarification on what Peter said.
>>
>> Remember that while Wien2k is more accurate than most other DFT codes,
>> it still has approximations with the form of the exchange-correllation
>> potential and in how the core wavefunctions are calculated. Hacking by
>> applying unphysical constraints so it will match experiments is wrong.
>> (Remember the story of the graduate student who matched all properties
>> of silicon by tuning the parameters of the DFT calculation for each
>> one so it was "right".)
>>
>> I would instead suggest that you look at better functionals for the
>> core wavefunctions, see Novak et al, Phys. Rev. B 67, 140403(R) (2003)
>> as well as the papers that cite it and the earlier paper by U. Lundin
>> and O. Eriksson, Int. J. Quantum Chem. 81, 247 (2001) and papers that
>> cite this. If you ask Peter or Pavel really nicely they may be able to
>> provide the code that uses this functional but you will almost
>> certainly have to do some coding work. This might not explain your
>> experimental results, and if it does not either the experiments are
>> wrong or we just don't have good enough theory yet for what you are
>> measuring, probably the latter.
>>
>>
> 

-- 
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Peter Blaha
Inst. Materials Chemistry, TU Vienna
Getreidemarkt 9, A-1060 Vienna, Austria
Tel: +43-1-5880115671
Fax: +43-1-5880115698
email: pblaha at theochem.tuwien.ac.at
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