Dear Yangchuan Li,
In principle the values of cut-off are fully transferable, for the same
kind of pseudo potential: As the highest Fourier components are required
due to more localised (ie fastly varying) electronic wave functions close
to the nuclei and there the changes going from one chemical environment to
a different one are small, in most cases the cut-off tested in one system
are indeed transferable (a reminder: using the same pseudo potential, so
the recommended values for the cut-off energies are not strictly element-
but even pseudo potential-specific).
About the k points, rather the bigger the system, less k points: That is
"hidden" in the word "reciprocal" space. Please have a look in the
discussions on the back-folding of k points etc. The bottom line is, to
get the same kind of accuracy in systems of different sizes the distance
between the k points should be more or less the same (as they are used to
approximate an integral). And if the cell in real space gets bigger, the
Brillouin zone gets smaller: Thus less k points needed if the distance
between the points is kept the same.
And the density depends on the kind of system: In general one needs more
points in metallic systems (in order to integrate the states close to the
Fermi level accurately) than in semi-conducting or insulating systems. But
if in the latter there are some more exotic bands somewhere needing higher
integration sampling in the reciprocal space, more k points are also
needed there. So shortly, no, one cannot give a general "preferred" number
of k points; also because that depends on the quantity one is looking at
(the structure converges faster than for example the phonon modes etc).
Greetings from Sleepy Paris,
apsi
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Ari Paavo Seitsonen / [email protected] / http://www.iki.fi/~apsi/
Ecole Normale Supérieure (ENS), Département de Chimie, Paris
Mobile (F) : +33 789 37 24 25 (CH) : +41 79 71 90 935
On Thu, 18 Feb 2016, Yangchuan Li wrote:
Hi everyone,
I want to ask whether a converged Ecut and smearing width can be used in
another model. I have a big model (functionalized carbon nanotube) and it's
time-consuming to do scf
calculation. I notice Stefano mentioned that Ecut is not strongly depended on
particular configuration. (In topic Technique for converging Ecut and K-points?)
So can I used small model like diamond, nitrogen gas to do convergence test and
then use the preferred parameter in my big model? Why I can or can't do that?
Also, if the dimension of the system is big. Is it necessary to select a very
dense k-mesh? Is there a preferred k-point density in reciprocal space?
Thank you for your time. Any comment will be appreciated.
Yangchuan Li
UT Austin | ME
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