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Dear Hud,<br>
<br>
I potentially see two sub-questions in your message:<br>
<br>
1) if I have a system with various C atoms, concurring to the same
K-edge but possibly slightly misaligned, how to align them with
respect to each other? Let me call this an "internal alignment" of
the spectrum.<br>
<br>
Here you have to consider that various C atoms may have different
C1s binding energy (that is, the 1s energy wrt vacuum level), as
well as a different alignment of the empty energy levels with
respect to the vacuum level. Since the photon energy is the
difference between the two quantities, both the core-level-shift and
such alignment have to be taken into account as a shift of the
xanes.dat files before summing them. This is what we do for aromatic
molecules and we described it in J. Phys. Chem. C, 2013, 117 (13),
pp 6632–6638 DOI: <a class="moz-txt-link-freetext" href="http://dx.doi.org/10.1021/jp312569q">http://dx.doi.org/10.1021/jp312569q</a><br>
If yours is a bulk system, the experimental C1s energy may be
referred to the Fermi energy rather than the vacuum level, and so in
the calculations.<br>
<br>
2) once a total spectrum is determined (with arbitrary energy
reference), or if you have just one C species, how to align this to
experiments?<br>
<br>
The spectra "shifted internally" are referred to an unknown
quantity, that is however a constant, that is the average core level
binding energy for your C atoms (see E_i^avg in Eq.4 in that JPCC
paper), that for C amounts to about 285eV (that is indeed the shift
you need to make a sensible comparison). So you are left with few
options, at least:<br>
<br>
- take such average core level binding energy from experiments
(C1s-XPS)<br>
- evaluate that with some theoretical approach (e.g., all-electron /
PAW)<br>
- assume an empirical shift (that would be the only empirical
parameter in your calculation)<br>
<br>
Kind regards,<br>
Guido<br>
<br>
<br>
<div class="moz-cite-prefix">On 05/08/2016 00:57, Hud Wahab wrote:<br>
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<p class="MsoPlainText">Hello QE community<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">I am trying to compare the xspectra
(5.3.0) and experimental absorption data at the carbon K-edge,
that is, the measured energies for carbon K-edge between 285
eV and 320 eV. How does one shift the xspectra simulation to
the absolute energy scale of the experiment? I think that
means getting an element-specific absolute value of the energy
shift. I understand that the calculated Fermi energy is one
quantity we need to shift by – but this value is nowhere near
the ~285 eV needed to make a sensible comparison.<o:p></o:p></p>
<p class="MsoPlainText"> <o:p></o:p></p>
<p class="MsoPlainText">And, if the absolute value was
element-specific, in this case for carbon, it presumably comes
from the generation of the pseudopotential and is hopefully
recorded in the pseudopotentil file (e.g.<o:p></o:p></p>
<p class="MsoPlainText">C.star1s-pbe-mt_gipaw.UPF). What are
the units of the energy given in this particular file?<o:p></o:p></p>
<p class="MsoPlainText"> <o:p></o:p></p>
<p class="MsoPlainText">“<PP_GIPAW_CORE_ORBITAL><o:p></o:p></p>
<p class="MsoPlainText"> 1 0 N L
1S eig: -25.48712189”<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">-Hud<o:p></o:p></p>
<p class="MsoPlainText">UNSW Canberra</p>
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<br>
Guido Fratesi<br>
Dipartimento di Fisica<br>
Universita` degli Studi di Milano<br>
Via Celoria 16, 20133 Milano<br>
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