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Dear Stefano,<br>
<br>
That is very helpful thank you, I will try out your strategy now.<br>
<br>
I did have one more question if I may ask it. Suppose I have single
unit cell FCC and I converge the energy cutoff and k-points. If I
then set up a 2x2x2 supercell of that unit cell, would I have to
repeat the energy cutoff and k-point convergence for the larger
supercell?<br>
<br>
All the best,<br>
<br>
Ben Palmer, Student @ University of Birmingham, UK<br>
<br>
<br>
<blockquote cite="mid:512B6E0F.7050408@sissa.it" type="cite">
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<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 moz-do-not-send="true" class="moz-txt-link-abbreviated" href="mailto:benpalmer1983@gmail.com">benpalmer1983@gmail.com</a>
To: <a moz-do-not-send="true" 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
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