[Pw_forum] Work function and void level using exchange-correlation potential

[stefano de gironcoli]

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

anyhow vxc should eventually die out and approach the 
"electrostatic-only" potential that gives therefore a better estimate 
for the workfunction

hope this helps

stefano

On 11/08/2013 04:27 PM, MARCO.PIVIDORI at phd.units.it wrote:
> Work function and void level using exchange-correlation potential
>
> Dear PW_forum users,
>
> 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.
>
> 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.
> (here a plot I have obtained for an Aluminum slab 
> https://dl.dropboxusercontent.com/u/38867209/Aluminum_6layer111slab_LDA.ps 
> )
>
> 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).
>
> 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
>
> I'm working with version 5.0.2 of qe (I've also tried with 4.3.1 
> obtaining same results)
>
> 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)
>
> 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?
>
> 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.
> Best regards,
>
> Marco
>
> ---
> Marco Pividori
> PhD Student
> Department of Physics
> University of Trieste
> Strada Costiera 11
> I-34151 Trieste
>
>
>
>
>
>
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