[Ifeffit] Ifeffit Digest, Vol 52, Issue 9

[Lena Fitting]

Dear Michel,

First of all thank you very much for your help with this!

One small comment: I thought that adding an imaginary part to the 
potential in EXCHANGE would effectively damp out waves at large 
distances. So, I was expecting that the XANES would "converge" at a 
certain cluster size, after which no changes of the fine structure would 
occur. If that's correct, does it mean that I should choose a larger Vi?

Thanks again for you help.

Lena



Michel Jaouen wrote:
>     Dear lena,
>
>> I have a question regarding the convergence of XANES calculations. I'm
>> currently focusing on the O-K edge of perovskite oxides, in particular
>> SrTiO3 and found that there are still fine-structure changes for large
>> (9.5?) clusters. Attached is my *.inp file for the 9.5? cluster and a
>> summary of the XANES for increasing cluster sizes from 6.5? to 9.5?. You
>> can see that there are still significant changes as the cluster size is
>> increased from 8.5 to 9.5?. In particular, I find a double peak feature
>> in the first 5eV after the edge onset, which is not observed
>> experimentally.
>
> It is a normal behavior: when increasing the cluster's size, you are 
> adding more MS paths and then it appears coresponding peaks. It is a 
> general trend for xanes:the measured signal is a probe up to a given 
> distance for MS, a distance you didn't know a priori. Thus the game is 
> to start from a rather small cluster, compute the xanes, compare to 
> the experiment. If some features are missing, increase the size and so 
> on till you obtain (if lucky) all experimental fine structures. On the 
> opposite, as soon as it appears fine structures which don't exist in 
> the experiment reduce the size.
>
>>  Do you have any suggestions how to obtain a better
>> convergence?
>>
>> Secondly, if you compare the simulated XANES with experimental results
>> (see attachment: EELS STO.png), I'm not able to match all three main
>> peaks, even by scaling the energy scale. Are the deviations I'm seeing
>> within the accuracy of FEFF or do I need to improve my *.inp file?
>
> I have been working a lot on STO and have many exchange about this 
> material with John Rehr. In fact you can obtain a quite good match 
> with the experiment (peaks' positions) if you reduce the lattice 
> parameter by 5% (RMULTIPLIER=0.95, see the attached figure). Of course 
> such a reduction is unphysical but from John's own words :"Also 
> regarding SrTiO3, I'm now thinking that the self-energy might
> be responsible for the need to use an RMULT 0.95." So either you the 
> Rmultiplier trick or you try the feff8.5 version that includes some 
> improvements to the self-energy (using low-loss and the sum rules for 
> normalization). In any case, feff has to much intensity at the edge 
> onset (sigma*) for light elements like Oxygen.
>
> Two other comments: why adding a real part vr=1.4eV? I don't find this 
> necessary here from my numerous calculations (it is responsible for 
> the hump at the bottom of the pi* which doesn't exist in the 
> experiment: a positive vr induces a red-shift for the spectrum, look 
> at the ldos). I suggest also not to use the default for lmax (-1), but 
> instead lmax=2 for all atoms (spd basis). You can also try to go up to 
> lmax=3 (spdf basis) because Sr is quite heavy and it can help in some 
> case (for pure Ni for instance, to reproduce the K edge, you must go 
> up to f states).
>
> I hope it can help you.
>
> Best regards,
>
>
>     Michel
> ------------------------------------------------------------------------
>
> _______________________________________________
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
>   


-- 
---------------------------------
Lena Fitting Kourkoutis
E13 Clark Hall
Cornell University
Applied and Engineering Physics
Ithaca, NY 14853

phone: 607-255-0654
e-mail: lf56 at cornell.edu
---------------------------------