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Hi Emine,<br>
<br>
Thank you for clearing up what I should be converging. I wasn't too
sure if I should be trying to converge the smearing and k-point
combinations to a single value. I'll give your technique a go, and
also incorporate the forces as mentioned by Nicola. I'm just on my
university library and should have Stefano's paper soon.<br>
<br>
All the best,<br>
<br>
Ben<br>
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<div style="direction: ltr; font-family: Tahoma; color: rgb(0, 0,
0); font-size: 10pt;">Hi Ben,<br>
<br>
the idea is to calculate the "converged energy" wrt "smearing
width."<br>
So, the procedure as I understand is<br>
1.choose a smearing width<br>
2.converge the energy at that smearing width with by choosing an
appropriate k mesh<br>
3.repeat it for other values of smearing width.<br>
<br>
You need to be quite generous with the range of smearing width
you explore.<br>
Your values span a very narrow range where smearing is <u>_very_</u>
small.<br>
Although it depends on the material, I find a range of 0.1 to
0.005 is generally a good starting point.<br>
Guess I would do calculations at around 0.1, 0.05, 0.03, 0.02,
0.01, 0.005 if i was exploring the correct behaviour here..<br>
<br>
An example:<br>
To understand if you have spanned a good range for the k point
grids you can afford,<br>
look for "merging lines":<br>
As you increase the smearing, the convergence is gonna get to be
more easily reached with even smaller k point meshes. so you
would be able to see<br>
* lines for 18, 16 to merge to same value lets say -154.062, at
width 0.005 ;<br>
* then as you increase smearing width further to 0.02 you would
see 18 16 14 12 mesh lines will all merge to same value of
-154.061,<br>
* and continuing, at width 0.1 all lines 18 16 14 12 8 4 will
converge to energy value -154.058<br>
<br>
that is what we mean by "converged energy" at each width:<br>
when k point lines merge it means that you have reached
convergence wrt k mesh for that width.<br>
<br>
in the above made-up example,<br>
smearing width versus converged energy would look like:<br>
<br>
0.005 -154.062 (18,16 merged)<br>
0.01 -154.062 (18 16 )<br>
0.02 -154.062 (18 16 14)<br>
0.03 -154.061 (18 16 14 12)<br>
0.05 -154.060 (18 16 14 12 8)<br>
0.1 -154.058 (18 16 14 12 8 4)<br>
<br>
so 0.02 width and 14 kmesh could be considered converged with
the given precision here.<br>
<br>
if you have chosen a very small width range (for the k grids you
can afford), you will see separate, flat lines as you do in your
figure instead. Anyways, next time such separate flat lines can
be your cue to explore wider ranges of smearing width.<br>
<br>
Note that in the above example, i have always reduced the energy
as i reduced the smearing width. this does not have to be so.
Indeed in Nicola's thesis and Stefano's paper on phonons w
metallic systems, you can see how different ways of smearing
would depend differently on the shape of the density of states
at the fermi level to determine this behaviour.<br>
<br>
Also note that in the above example i have used energy as a
convergence criterion but you could have as well used forces as
Nicola suggested.<br>
<br>
You already have Nicola's thesis link, and here is Stefano's
paper: <a moz-do-not-send="true"
href="http://prb.aps.org/abstract/PRB/v51/i10/p6773_1"
target="_blank">
http://prb.aps.org/abstract/PRB/v51/i10/p6773_1</a><br>
<br>
best<br>
emine kucukbenli, postdoc at theos, epfl, switzerland<br>
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