[Ifeffit] Ifeffit Digest, Vol 52, Issue 9
Michel Jaouen
Michel.Jaouen at univ-poitiers.fr
Mon Jun 18 11:15:34 CDT 2007
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
--
__________________________________________
Michel Jaouen
Universite de Poitiers - UFR Sciences - SP2MI
LMP UMR 6630-CNRS
Boulevard Pierre et Marie Curie - Teleport 2
BP 30179
86962 Futuroscope - Chasseneuil Cedex
France
Tel : (33) 5 49 49 67 37
Fax : (33) 5 49 49 66 92
e-mail : Michel.Jaouen at univ-poitiers.fr
__________________________________________
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