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Dear Maxim,<br>
<br>
At the moment I'm calculating the bulk modulus and elastic constants
of a few metals, and will be relaxing supercells with defects after
this. In the literature I've read through, I've seen quite a range
of k-points used, from paper to paper, ranging up to 32x32x32. One
of the assessors at my University was surprised I was using as many
as 6x6x6 k-points, so I really wanted find out how to select the
best parameters for those reasons too. Our computer is also very
busy most of the time (this is probably the same for any university
computer), so picking the right parameters is important to me in
that respect too.<br>
<br>
All the best,<br>
<br>
Ben<br>
<br>
<blockquote
cite="mid:CAORFTiEWJ5kYjdhChFF2wf_DSZM5xDzBofBZf00y_CKte2Lr9Q@mail.gmail.com"
type="cite">Dear Ben,
<div><br>
</div>
<div>I would also like to contribute my 5 cents to the discussion.</div>
<div>There are two main questions to answer prior to any
convergence study.</div>
<div><br>
</div>
<div>1) What quantity is of interest?</div>
<div><br>
</div>
<div>If you can answer this question, you can save a lot of time
and computational resources, because different</div>
<div>quantities require different parameters (cut-offs,
BZ-sampling, etc.) to converge. Converging total energies</div>
<div>is not always a good idea.</div>
<div><br>
</div>
<div>2) How much precision do I really need?</div>
<div><br>
</div>
<div>Answering this question depends on the problem you are trying
to solve. For instance, if you are trying to</div>
<div>identify which of two phases of some substance is most stable
at 0K (according to DFT), and the difference</div>
<div>in the free energies of these phases is big, then you would
need rather coarse parameters to figure out the answer</div>
<div>to your problem. It might happen though, that the difference
is tiny, then you would have to tune the parameters</div>
<div>till a reliable distinction is achieved.</div>
<div><br>
</div>
<div>Best regards,</div>
<div>Maxim.</div>
<div><br>
</div>
<div><br>
<div class="gmail_quote">2013/2/25 Ali KACHMAR <span dir="ltr"><<a
moz-do-not-send="true"
href="mailto:kachmar_ali@hotmail.fr" target="_blank">kachmar_ali@hotmail.fr</a>></span><br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">
<div>
<div dir="ltr">
Dear Stefano,<br>
<br>
That's great. In any case, I learnt too much from you
and from many others on this forum.<br>
<br>
Thank you,<br>
Ali<br>
<br>
<div>> Date: Mon, 25 Feb 2013 15:05:14 +0100<br>
> From: <a moz-do-not-send="true"
href="mailto:degironc@sissa.it" target="_blank">degironc@sissa.it</a><br>
> To: <a moz-do-not-send="true"
href="mailto:pw_forum@pwscf.org" target="_blank">pw_forum@pwscf.org</a><br>
> Subject: Re: [Pw_forum] Technique for converging
Ecut and K-points?
<div>
<div class="h5"><br>
> <br>
> Dear All,<br>
> My previous post was actually more intended
as an answer to Ben <br>
> Palmer question than a comment to<br>
> Ali Kachmar contribution. Sorry.<br>
> best regards,<br>
> stefano<br>
> <br>
> <br>
> On 02/25/2013 02:58 PM, Stefano de Gironcoli
wrote:<br>
> > Dear Ali Kachmar,<br>
> ><br>
> > convergence w.r.t. ecutwfc (and ecutrho)
and convergence w.r.t. <br>
> > k-points sampling are rather independent
issues and can be tested to a <br>
> > large extent separately<br>
> ><br>
> > - convergence w.r.t. ecutwfc and ecutrho
is a property depending on <br>
> > the highest Fourier components that are
needed to describe the <br>
> > wavefunctions and the density of your
system. his depends on the <br>
> > pseudopotentials that are present in the
calculation and do not depend <br>
> > strongly, for a given set of
pseudopotentials, on the particular <br>
> > configuration because it depends mostly
on the behaviour of the wfc in <br>
> > the core region which is quite
insensitive (in terms of shape) on the <br>
> > environment.<br>
> > So each pseudopotential has a required
cutoff. An upperbound to this <br>
> > value can be determined from any system
that contains that pseudo.<br>
> > The cutoff needed for a system
containing several species is the <br>
> > highest among those needed for each
element.<br>
> > Moreover, in US pseudo or PAW the charge
density has contributions <br>
> > from localized terms that may (an
usually do in USPP) require quite <br>
> > higher cutoff than the one needed for
psi**2 (4*ecutwfc) ... hence the <br>
> > possibility to vary and test
independently for ecutrho ...<br>
> ><br>
> > My recommended strategy to fix ecutwfc
and ecutrho is to perform total <br>
> > energy (and possibly, force and stress)
covergence test increasing <br>
> > ecutwfc keeping ecutrho at its default
vaule (=4*ecutwfc) until <br>
> > satisfactory stability is reached
(typically ~1 mry/atom in the <br>
> > energy, 1.d-4 ry/au in the forces, a
fraction of a KBar in the stress) <br>
> > ... this fixes the converged value of
ecutrho to 4 times the <br>
> > resulting ecutwfc.<br>
> > Now keeping this value for ecutrho one
can try to reduce ecutwfc and <br>
> > see how much this can be done without
deteriorating the convergence.<br>
> ><br>
> > -convergence with respect to k-points is
a property of the band <br>
> > structure.<br>
> > I would study it after the
ecutwfc/ecutrho issue is settled but some <br>
> > fairly accurate parameters can be
obtained even with reasonable but <br>
> > not optimal cutoff parameters.<br>
> ><br>
> > There is a big difference between
convergence in a band insulator or <br>
> > in a metal.<br>
> ><br>
> > In an insulator bands are completely
occupied or empty across the BZ <br>
> > and charge density can be written in
terms of wannier functions that <br>
> > are exponentially localized in real
space.<br>
> > Hence the convergence w.r.t the density
of point in the different <br>
> > directions in the BZ should be
exponentially fast and anyway quite <br>
> > quick...<br>
> ><br>
> > In a metal the need to sample only a
portion of the BZ would require <br>
> > an extremely dense set of k points in
order to locate accurately the <br>
> > Fermi surface. This induces to introduce
a smearing width that smooth <br>
> > the integral to be performed... the
larger the smearing width, the <br>
> > smoother the function, and the faster
the convergence results...<br>
> > however the larger the smearing width
the farther the result is going <br>
> > to be from the accurate, zero smearing
width, result that one would <br>
> > desire.<br>
> > Therefore different shapes fro the
smearing functions have been <br>
> > proposed to alleviate this problem and<br>
> > Marzari-Vanderbilt and Methfessel-Paxton
smearing functions give a <br>
> > quite mild dependence of the (k-point
converged) total energy as a <br>
> > function of the smearing width thus
being good choices for metals.<br>
> ><br>
> > My recommended strategy for fix the
k-point sampling in metals is<br>
> > 1) chose the smearing function type (mv
or mp, recomended)<br>
> > 2) for decreasing values of the smearing
width (let's say from an high <br>
> > value of 0.1 ry = 1.36 eV to a low value
of 0.01 - 0.005 ry = <br>
> > 0.136-0.068 eV if feasable) CONVERGE the
total energy w.r.t to <br>
> > smearing well within the global desired
tolerance (of 1 mry/atom, for <br>
> > instance)<br>
> > 3) by examining the behaviour of the
CONVERGED Energy vs smearing <br>
> > width curve E(sigma) identify the
smearing width for which E(sigma) is <br>
> > within tolerance w.r.t. E(sigma==0)
keeping in mind that for <br>
> > methfessel-paxton E(sigma) ~ E(0) +
A*sigma**4 + o(sigma**6) while for <br>
> > marzari-vanderbilt the dependence is
more likely E(sigma) ~ E(0) <br>
> > +A*sigma**3 + o(sigma**4).<br>
> > 4) select that value of the smearing
width and the smallest set of <br>
> > k-points for which this is converged.<br>
> ><br>
> > HTH<br>
> ><br>
> > stefano<br>
> ><br>
> ><br>
> ><br>
> > On 02/24/2013 06:54 PM, Ali KACHMAR
wrote:<br>
> >> Hi,<br>
> >><br>
> >> as far as I know, there is no any
techinques for choosing ecut and <br>
> >> k-points. Please have a look at the
pwscf archive and make up a <br>
> >> conclusion.<br>
> >><br>
> >> Best,<br>
> >> Ali<br>
> >><br>
> >>> Date: Sat, 23 Feb 2013 19:55:51
+0000<br>
> >>> <a moz-do-not-send="true"
href="mailto:From%3Abenpalmer1983@gmail.com"
target="_blank">From:benpalmer1983@gmail.com</a><br>
> >>> <a moz-do-not-send="true"
href="mailto:To%3Apw_forum@pwscf.org"
target="_blank">To:pw_forum@pwscf.org</a><br>
> >>> Subject: [Pw_forum] Technique
for converging Ecut and K-points?<br>
> >>><br>
> >>> Hi everyone,<br>
> >>><br>
> >>> I just wanted to ask if users
have any techniques for choosing ecut and<br>
> >>> k-points? I've read that one way
would be to start with a high number<br>
> >>> of k-points and high energy
cutoff, and use that energy as an almost<br>
> >>> true value. Then adjust k-points
and energy cutoff from a lower<br>
> >>> number/cutoff until it converges
to the true value. Would you try to<br>
> >>> converge energy cutoff first, or
k-points? Does it matter which you<br>
> >>> converge first?<br>
> >>><br>
> >>> Thanks<br>
> >>><br>
> >>> Ben Palmer<br>
> >>> Student @ University of
Birmingham<br>
> >>>
_______________________________________________<br>
> >>> Pw_forum mailing list<br>
> >>> <a moz-do-not-send="true"
href="mailto:Pw_forum@pwscf.org" target="_blank">Pw_forum@pwscf.org</a><br>
> >>> <a moz-do-not-send="true"
href="http://pwscf.org/mailman/listinfo/pw_forum"
target="_blank">http://pwscf.org/mailman/listinfo/pw_forum</a><br>
> >><br>
> >><br>
> >><br>
> >>
_______________________________________________<br>
> >> Pw_forum mailing list<br>
> >> <a moz-do-not-send="true"
href="mailto:Pw_forum@pwscf.org" target="_blank">Pw_forum@pwscf.org</a><br>
> >> <a moz-do-not-send="true"
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target="_blank">http://pwscf.org/mailman/listinfo/pw_forum</a><br>
> ><br>
> ><br>
> <br>
>
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</div>
<br>
<br clear="all">
<div><br>
</div>
-- <br>
Best regards, Max Popov<br>
Ph.D. student<br>
Materials center Leoben (MCL), Leoben, Austria.
</div>
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