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Dear Jihoon Park,<br>
<br>
Yes there is a method, which is the one you are using. <br>
More precisely, it always depends on the property you are interested
in. <br>
For instance, the kmesh for optical properties will be denser than
for total energy (or a DOS). <br>
Thus, we should always check the convergency of the property we are
calculating in terms of k-points, at least. <br>
<br>
For magnetic properties, you need a good k-mesh for sure. One
solution could be to take the smallest unit cell (ferromagnetic
order) with the full-symmetry of the crystal and to check the k-mesh
convergency. Then you will know how many k-points you need in the
FULL-Brillouin ZONE (FBZ) to properly describe the FM state . <br>
<br>
Based on this information, you can extrapolate for a larger cell. <br>
For instance, if you need 800 k-points in the FBZ in the
crystallographic cell calculation (FM-state). <br>
For a supercell with a volume, which is 8 times bigger, you will
need 100 k-points. <br>
<br>
Such an approach is a pragmatic one and you always need to check the
coherence of your assumptions. <br>
In particular, the k-mesh also depends on the magnetic order (to a
smaller extent than volume of the cell). <br>
<br>
Finally, if your interest is to compare different magnetic orders,
it means that you are interested in the convergency of energy
differences. <br>
In many cases, "energy difference" converge faster than "total
energy" ... It means that your convergence criteria could be less
restrictive. But here also you have to do it with care. <br>
<br>
Best Regards<br>
<br>
Xavier<br>
<br>
<br>
<br>
<br>
<br>
<br>
Le 25/10/2012 18:03, Jihoon Park a écrit :
<blockquote
cite="mid:CAJT6voRHOxuyaYZ3wkg6NS7Z2_PCkSNvcjzECf1a8OtXX8ygmw@mail.gmail.com"
type="cite">Dear Users,
<div><br>
</div>
<div><br>
</div>
<div>I am trying to calculate magnetic properties for a large
unitcell (64 atoms).</div>
<div>For this kind of large unitcells, it take very long time to
do the KPOINTS convergence test.</div>
<div>Should I anyway "have to" do the KPOINTS convergence test or
is there any other method to find a good KPOINTS?</div>
<div><br>
</div>
<div><br>
</div>
<div>All my best,</div>
<div>Jihoon Park</div>
<br>
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<br>
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