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<div class="moz-cite-prefix">vxc decays slowly (like rho^1/3 in lda
for instance), moreover the density never really dies
exponentially in the vacuum but starts to have little oscillations
due to insufficient cutoff... in order to reduce these
oscillations (typically stronger in GGA) one can increase ecutrho<br>
<br>
anyhow vxc should eventually die out and approach the
"electrostatic-only" potential that gives therefore a better
estimate for the workfunction<br>
<br>
hope this helps<br>
<br>
stefano<br>
<br>
On 11/08/2013 04:27 PM, <a class="moz-txt-link-abbreviated" href="mailto:MARCO.PIVIDORI@phd.units.it">MARCO.PIVIDORI@phd.units.it</a> wrote:<br>
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<title>Work function and void level using exchange-correlation
potential</title>
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<p><font size="2">Dear PW_forum users,<br>
<br>
I was working with the post-processing of qe to obtain the
work functions for some metallic slabs (aluminum, gold..); as
also explained in one of the examples of PP, I've plotted the
potential of the slab as a function of the out-of-plane axis
to obtain the potential in the void region.<br>
<br>
My problem rises up when I compare the total potential (local
+ Hartree + XC as in plot_num=1 ) with the electrostatic one
(local + Hartree as in plot_num=11 ). The two potentials are
quite different from each other even in the void region, where
the XC contribute should(?) be zero.<br>
(here a plot I have obtained for an Aluminum slab <a
moz-do-not-send="true"
href="https://dl.dropboxusercontent.com/u/38867209/Aluminum_6layer111slab_LDA.ps">https://dl.dropboxusercontent.com/u/38867209/Aluminum_6layer111slab_LDA.ps</a>
)<br>
<br>
I understand that the XC contributions are more long-ranged
than the electrostatic ones and this explains the different
shape of the two curves, what I would like to understand is
why is there this constant shift when the potentials reach the
plateau (and, more important, which one is better to trust).
The shift is certainly non-negligible in the systems that I've
analyzed (0.41eV in Al_LDA, 0.44eV in Al_GGA, 0.19eV in Au_LDA
and 1.12eV in Au_GGA) and the total potential shows a more
"noisy" behaviour with an odd curvature (the system are all
neutral).<br>
<br>
I tried several cell size to see if the problem is related to
the amount of void but it seems unrelated (in the plot that
I've linked to you the slab distance is set to 30 A, that I
think is quite enough to avoid any periodic effect). Same
thing for the k_points sampling<br>
<br>
I'm working with version 5.0.2 of qe (I've also tried with
4.3.1 obtaining same results)<br>
<br>
Since the problem seems to be related to the exchange part I
also tried other kinds of pseudos.. the above data comes from
US PP, so i tried a norm conserving one for Al_LDA and the
shift got reduced by an order of magnitude (the strange bent
behaviour still remained)<br>
<br>
My question is.. has anyone ever dealt with this kind of
issue? it's a known problem or it has just popped up in my
calculations? <br>
<br>
I hope I've made myself clear and I thank you in advance for
your consideration. If there is something else that you need I
will try to produce it to you as soon as possible.<br>
Best regards,<br>
<br>
Marco<br>
<br>
---<br>
Marco Pividori<br>
PhD Student<br>
Department of Physics<br>
University of Trieste<br>
Strada Costiera 11<br>
I-34151 Trieste<br>
<br>
<br>
<br>
<br>
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