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Re: xstep and estep parameters of XANES card
Dear Lydia,
I would like to clarify the energy grid question.
The regular step in k-space is typical for EXAFS applications.
In this case k=delatk*(i-i0)
E(i) = const* k**2
where i0 correspond to Fermi level, and k values you can see in
column 3 of xmu.dat file. k=0 correspond to Fermi level.
This grid is fine for high energies, however gives too many points
around k=0. Aroud k=0 we use grid regular in energy
E=E0 + (i-i0)* deltae.
Notice that this grid will give too many points for high energies.
The transition from one grid to the other is made where
the energy step is about the same. You can see where this transition
happens by looking at column 3 of xmu.dat files.
The maximum number of energy points written in xmu.dat file is 100.
The additional 18 points are on pure Imaginary axis above the Fermi level.
They are needed to account for the proper broadening.
Best wishes
Alex Ankudinov
On Thu, 8 Jul 2004 ALNemir@aol.com wrote:
> Dear FEFF8 users,
>
> I am looking at the XANES of LiCoO2, Co edge. At some point I wanted to get a
> better resolution of a part of the plot, but it is not clear to me how to do
> that. It seems that the xstep and estep parameters of the XANES card would
> determine that. It is stated in the FEFF8 manual that "xstep specifies the size
> of the output k grid far from the edge" while "the grid at the edge will be
> regular in energy with a step size of estep." I was wondering where these
> respective regions begin and end (and if there are any additional regions in between
> them), and whether those points are fixed or floating. Though I played around
> with the above-mentioned parameters I was not able to find a definitive
> answer. Please see the enclosed pdf file for a plot of the xmu.dat files with
> different values of xstep and estep.
>
> *On a related note, please correct me if I am wrong, but I was assuming that
> the number of points in the XANES calculation output file, xmu.dat, is related
> to the number of "energy points" specified in the log3.dat file of the FMS
> calculation.
>
> Log3.dat:
> Number of energy points = 118
> Doing FMS for a cluster of 87 atoms around iph = 0
> Please, wait (updates every 20 points) ...
> FMS matrix (LUD) at point 20, number of state kets =1224
> FMS matrix (LUD) at point 40, number of state kets =1224
> FMS matrix (LUD) at point 60, number of state kets =1224
> FMS matrix (LUD) at point 80, number of state kets =1224
> FMS matrix (LUD) at point 100, number of state kets =1224
> Done with module 3: FMS.
>
> -The number of those "energy points" seems to have a limit of 120. Is there a
> reason for that?
> -It was 118 for most of the calculations plotted in the enclosed file, except
> for the one with parameters xmax=3, xstep=.07, estep=.01, in which was 72.
> Why is that?
> -From the log file presented above it is not clear if the last 18 points are
> calculated and written into an output file.
> -Whether the above assumption * is correct or not, I would like to know where
> the number of points in the xmu.dat file comes from. It is 100 for the
> 118-energy-points cases and 54 for the 72-energy-point case.
>
> Thank you,
>
> Lydia Nemirovsky
> Research Assistant
> EXAFS Laboratory
> Department of Physics and Astronomy
> Hunter College
>