<html>
<head>
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
</head>
<body text="#000000" bgcolor="#FFFFFF">
<div class="moz-cite-prefix">Dear Ali Kachmar,<br>
<br>
convergence w.r.t. ecutwfc (and ecutrho) and convergence w.r.t.
k-points sampling are rather independent issues and can be tested
to a large extent separately<br>
<br>
- convergence w.r.t. ecutwfc and ecutrho is a property depending
on the highest Fourier components that are needed to describe the
wavefunctions and the density of your system. his depends on the
pseudopotentials that are present in the calculation and do not
depend strongly, for a given set of pseudopotentials, on the
particular configuration because it depends mostly on the
behaviour of the wfc in the core region which is quite insensitive
(in terms of shape) on the environment. <br>
So each pseudopotential has a required cutoff. An upperbound to
this value can be determined from any system that contains that
pseudo.<br>
The cutoff needed for a system containing several species is the
highest among those needed for each element. <br>
Moreover, in US pseudo or PAW the charge density has contributions
from localized terms that may (an usually do in USPP) require
quite higher cutoff than the one needed for psi**2 (4*ecutwfc) ...
hence the possibility to vary and test independently for ecutrho
...<br>
<br>
My recommended strategy to fix ecutwfc and ecutrho is to perform
total energy (and possibly, force and stress) covergence test
increasing ecutwfc keeping ecutrho at its default vaule
(=4*ecutwfc) until satisfactory stability is reached (typically
~1 mry/atom in the energy, 1.d-4 ry/au in the forces, a fraction
of a KBar in the stress) ... this fixes the converged value of
ecutrho to 4 times the resulting ecutwfc.<br>
Now keeping this value for ecutrho one can try to reduce ecutwfc
and 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
structure. <br>
I would study it after the ecutwfc/ecutrho issue is settled but
some fairly accurate parameters can be obtained even with
reasonable but not optimal cutoff parameters. <br>
<br>
There is a big difference between convergence in a band insulator
or in a metal.<br>
<br>
In an insulator bands are completely occupied or empty across the
BZ and charge density can be written in terms of wannier functions
that are exponentially localized in real space.<br>
Hence the convergence w.r.t the density of point in the different
directions in the BZ should be exponentially fast and anyway quite
quick...<br>
<br>
In a metal the need to sample only a portion of the BZ would
require an extremely dense set of k points in order to locate
accurately the Fermi surface. This induces to introduce a
smearing width that smooth the integral to be performed... the
larger the smearing width, the smoother the function, and the
faster the convergence results...<br>
however the larger the smearing width the farther the result is
going to be from the accurate, zero smearing width, result that
one would desire.<br>
Therefore different shapes fro the smearing functions have been
proposed to alleviate this problem and<br>
Marzari-Vanderbilt and Methfessel-Paxton smearing functions give
a quite mild dependence of the (k-point converged) total energy as
a 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 value of 0.1 ry = 1.36 eV to a low value of 0.01 - 0.005 ry
= 0.136-0.068 eV if feasable) CONVERGE the total energy w.r.t to
smearing well within the global desired tolerance (of 1 mry/atom,
for instance)<br>
3) by examining the behaviour of the CONVERGED Energy vs smearing
width curve E(sigma) identify the smearing width for which
E(sigma) is within tolerance w.r.t. E(sigma==0) keeping in mind
that for methfessel-paxton E(sigma) ~ E(0) + A*sigma**4 +
o(sigma**6) while for marzari-vanderbilt the dependence is more
likely E(sigma) ~ E(0) +A*sigma**3 + o(sigma**4).<br>
4) select that value of the smearing width and the smallest set of
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>
</div>
<blockquote cite="mid:BAY173-W51F4F91B9004708E44B5EFEF20@phx.gbl"
type="cite">
<pre wrap="">Hi,
as far as I know, there is no any techinques for choosing ecut and k-points. Please have a look at the pwscf archive and make up a conclusion.
Best,
Ali
</pre>
<blockquote type="cite">
<pre wrap="">Date: Sat, 23 Feb 2013 19:55:51 +0000
From: <a class="moz-txt-link-abbreviated" href="mailto:benpalmer1983@gmail.com">benpalmer1983@gmail.com</a>
To: <a class="moz-txt-link-abbreviated" href="mailto:pw_forum@pwscf.org">pw_forum@pwscf.org</a>
Subject: [Pw_forum] Technique for converging Ecut and K-points?
Hi everyone,
I just wanted to ask if users have any techniques for choosing ecut and
k-points? I've read that one way would be to start with a high number
of k-points and high energy cutoff, and use that energy as an almost
true value. Then adjust k-points and energy cutoff from a lower
number/cutoff until it converges to the true value. Would you try to
converge energy cutoff first, or k-points? Does it matter which you
converge first?
Thanks
Ben Palmer
Student @ University of Birmingham
_______________________________________________
Pw_forum mailing list
<a class="moz-txt-link-abbreviated" href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a>
<a class="moz-txt-link-freetext" href="http://pwscf.org/mailman/listinfo/pw_forum">http://pwscf.org/mailman/listinfo/pw_forum</a>
</pre>
</blockquote>
<pre wrap="">
</pre>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<br>
<pre wrap="">_______________________________________________
Pw_forum mailing list
<a class="moz-txt-link-abbreviated" href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a>
<a class="moz-txt-link-freetext" href="http://pwscf.org/mailman/listinfo/pw_forum">http://pwscf.org/mailman/listinfo/pw_forum</a></pre>
</blockquote>
<br>
</body>
</html>