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<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;">Dear all users of
QE,</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"><br style="">
</div>
<div class="yiv4205984502" id="yiv4205984502yui_3_16_0_1_1405007344016_8606" style="margin-bottom:0in;line-height:100%;">I have recently
started using XSpectra package to calculate an X-ray absorption
spectrum (XANES to be more specific) for GaMnAs material K-edge of
Mn. Currently, I work with the simplest model: cubic structure, 8
atoms in total: 1 Mn, 3 Ga and 4 As, no core-holes, etc.</div>
<div id="yiv4205984502yui_3_16_0_1_1405007344016_8652" class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"><br style="" class="yiv4205984502">
</div>
<div id="yiv4205984502yui_3_16_0_1_1405007344016_8651" class="yiv4205984502" style="margin-bottom:0in;line-height:100%;">After setting up the
model, I encountered a feature in the spectrum, which I am puzzled
with:</div>
<div id="yiv4205984502yui_3_16_0_1_1405007344016_8650" class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"><br style="" class="yiv4205984502">
</div>
<div id="yiv4205984502yui_3_16_0_1_1405007344016_8649" class="yiv4205984502" style="margin-bottom:0in;line-height:100%;">It seems that the
spectrum is more-or-less OK near fermi level (I can cut occupied
states). Then, from 0 eV up to about 15-17 eV, the spectrum has
correct positions of 2 peaks at around 9 and 13.5 eV (the ratio
between them is 1.0, however, as oppose to the experimental value of
1.2; still I can leave with that and maybe correct it by changing
gamma from 1.6 to slightly higher values).</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"><br style="" class="yiv4205984502">
</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;">However, after
reaching the maximum at 13.5 eV, the spectrum falls down steadily
(like 1/x) and relatively quickly. The maximum value at 13.5 eV was
0.0013 and at 40 eV it is less than 0.0001. The experimental data
instead would oscillate around value 0.0012 (which is a usual
behavior for a spectrum).</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"><br style="" class="yiv4205984502">
</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;">I tried several
things (just in case):</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"> - I increased
number of k-points in SCF calculations;</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"> - I increased
number of k-points in XSpectra calculations;</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"> - I increased
number of bands in SCF calculations;</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;">In all these cases
the spectra were the same (in numerical sense, of course)!!!</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"><br style="" class="yiv4205984502">
</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;">I also changed the
gamma coefficient from very low (0.01), to reasonable values (0.7 and
1.6), to very high (5). The gamma coefficient affects the
calculations as expected, but could not “lift up” the tail at all. At the
very high value the spectrum becomes broad one with one maximum only,
i.e. as expected, but the tail keeps falling down...<br></div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"><br style="" class="yiv4205984502">
</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;">So, my question is
whether anybody experienced such an effect before and what should I do in
order to correct it.
</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"><br style="" class="yiv4205984502">
</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;">Thank you in advance
for any hints/comments!</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"><br style="" class="yiv4205984502">
</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;">Yevgen Melikhov</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"><br>Lab of X-ray and Electron Microscopy Research<br style="" class="yiv4205984502">
</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;">Institute of
Physics, PAS</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;">Warsaw, Poland</div>
<div class="yiv4205984502" style="margin-bottom:0in;line-height:100%;"><br style="" class="yiv4205984502">
</div>
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