[Wien] A question about the value of energy loss in TELNES.2

Mon Apr 9 19:49:59 CEST 2007

Hi Ruijuan,

Wien2k (or any other comparable code) cannot accurately calculate threshold energies.  This is not a problem of numerical accuracy, but simply of formalism (we calculate one electron states, while EELS is a two particle problem).

Say you're doing a K edge, you can find the energy of the 1s orbital w.r.t. the Fermi level in the output of WIEN2k, and add a small amount if the edge in case.elnes doesn't start at 0 but a little higher ; but this will only give you a very rough idea of where the edge is.

If you do a core hole calculation, you will obtain a different (lower) threshold energy.  But it will also be off.
Supposedly, a calculation with a half core hole ("Slater transition state") should give you the correct threshold energy (paper by Paxton a few years ago - can't remember the exact reference now).  But I'm not sure that it actually does in our calculations. (Anyone?)

The number in case.innes is there so that we can calculate the q-vectors for the cross section accurately.  (It doesn't matter if it's off by a small fraction of a few eV.)

I stress that these are not limitations of the TELNES2 and WIEN2k implementation, but simply of the formalism that WIEN2k uses.

On the bright side, energy differences tend to be fairly accurate.  Eg., the L23-splitting came out well in the calculations that I've done ; and I also expect reasonably good chemical shifts.

To summarize : believe energy differences, but not absolute threshold energies calculated by WIEN2k+TELNES2.  Just shift to align with experiment.

Kevin.

Kevin Jorissen
University of Washington
Dept. of Physics
Box 351560
Seattle, WA-98195-1560
U.S.A.

phone +1 206 543 3904 
fax +1 206 685 0635 
e-mail kevin.jorissen at ua.ac.be 
web page http://fraangelico.phys.washington.edu/~jorissen

________________________________

From: wien-bounces at zeus.theochem.tuwien.ac.at on behalf of Ruijuan Xiao
Sent: Sun 4/8/2007 19:58
To: wien
Subject: [Wien] A question about the value of energy loss in TELNES.2

Dear all,

When I use the TELNES.2 to calculate energy loss near edge spectrum, I found that the value of the energy loss(x axis) in the simulated spectrum begins from "0". In the past, I always shift this x axis and make the onset of the theoretical spectra to well align with the experimental one, and make some comparison between them. I do not think there is any problem.

But recently, someone asked me about the meaning of the x axis. He emphasized that the value of energy loss is the difference between the energies of final and initial states, so there should be an absolute value calculated from the program, and any shift of the theoretical energy loss to compare with the experiments does not make sense. I do not know how to explain it to him. So I wonder why the program gives a theoretical spectrum starting from zero instead of calculating the absolute value of energy loss. Is it because some initial states are core states, their energies can not be calculated as accurately as valence states?

I also searched in this mailing list, and found some statements related to this question. It says "the '0' of the elnes spectrum corresponds to the threshold energy that you specify in case.innes.  So the 'real' energy loss is given by that on the x-axis, plus the threshold energy of case.innes.". But it seems that the threshold energy specified in case.innes is given by the user, so it can not reflect the real energy loss of the spectrum.

I am looking forward to your reply. Thank you very much in advance.

Best regards,

Sincerely,
Ruijuan

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