<div dir="ltr"><div>Dear Nicola,</div><div><br></div><div>thanks a lot, a very interesting read!</div><div><br></div><div>Best,</div><div>Chris <br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Sep 7, 2018 at 2:58 PM, Nicola Marzari <span dir="ltr"><<a href="mailto:nicola.marzari@epfl.ch" target="_blank">nicola.marzari@epfl.ch</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><br>
<br>
<br>
Hi Christoph,<br>
<br>
<br>
a dual of 2 is cutting a lot of corners for norm-conserving pseudopotentials (where a dual of 4 is mathematically perfect,<br>
and one of ~3 could actually work).<br>
<br>
Convergence in the total energy for norm-conserving at dual of 4<br>
is strictly variational, but also not very interesting as a criterion.<br>
<br>
The phonon frequencies are an interesting criterion, but are not variational.<br>
<br>
So, there is nothing fool proof - but a preponderance of evidence<br>
helps feeling better - this is discussed at length here:<br>
<a href="https://arxiv.org/abs/1806.05609" rel="noreferrer" target="_blank">https://arxiv.org/abs/1806.056<wbr>09</a><br>
<br>
<br>
nicola<div><div class="h5"><br>
<br>
<br>
<br>
On 07/09/2018 10:43, Christoph Wolf wrote:<br>
</div></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div class="h5">
Dear all,<br>
<br>
I am afraid that this is a very basic question but I will ask it anyway in the hopes of some pointers. I have recently studied the convergence behavior of a set (Mg and O) pseudos with respect to the phonon frequencies and I encounter a behavior that I quite not understand.<br>
<br>
I study the convergence of total energy and the highest phonon frequency at q=(0.5,0.5,0.5). I vary the dual=ecutrho/ecutwfc=2,4,8.<br>
<br>
for the norm-conserving pseudos (standard dual is 4 here) energy converges (monotonously) at a ecutwfc=50 irrespective of the dual. however the phonon frequency only converges (strongly non-monotonously, i.e. in a zig-zag pattern) if (i) ecutwfc=100 and dual 8 OR (ii)ecutwfc=200 and dual 4<br>
<br>
for the PSLibrary 1.0.0 pseudos the total energy converges (monotonously) at ecutwfc=50 for all duals but interestingly so does the phonon frequency (in excellent agreement with the converged norm.-cons.!). Varying the dual from 2 to 8 leaves the phonon frequencies virtually unchanged. the suggested hardest cutoff for the pseudos (from the file) is Mg: 97/398 - higher than what I found.<br>
<br>
Now I have read that for phonons and US/PAW often a dual of 8-12 is advised (I think the example is for metals not an insulator as MgO) but I was curious if there is any "fool proof" method to ensure the convergence whilst not risking falling in a "local minima" of a phonon frequency, for example?<br>
<br>
Thanks for reading all the way down to hear, your help is greatly appreciated!<br>
<br>
Best,<br>
Chris<br>
<br>
-- <br>
Postdoctoral Researcher<br>
Center for Quantum Nanoscience, Institute for Basic Science<br>
Ewha Womans University, Seoul, South Korea<br>
<br>
<br></div></div>
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</font></span></blockquote><span class="HOEnZb"><font color="#888888">
<br>
<br>
-- <br>
------------------------------<wbr>------------------------------<wbr>----------<br>
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL<br>
Director, National Centre for Competence in Research NCCR MARVEL, EPFL<br>
<a href="http://theossrv1.epfl.ch/Main/Contact" rel="noreferrer" target="_blank">http://theossrv1.epfl.ch/Main/<wbr>Contact</a> <a href="http://nccr-marvel.ch/en/project" rel="noreferrer" target="_blank">http://nccr-marvel.ch/en/proje<wbr>ct</a><br>
</font></span></blockquote></div><br><br clear="all"><br>-- <br><div class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr">Postdoctoral Researcher<br>Center for Quantum Nanoscience, Institute for Basic Science<br>Ewha Womans University, Seoul, South Korea<blockquote type="cite" style="font-size:12.8px"><div dir="ltr"><div><div dir="ltr"></div></div></div></blockquote></div></div>
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