QuantumATK Forum
QuantumATK => General Questions and Answers => Topic started by: Luis M. Villamagua C. on November 21, 2013, 15:07
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Hi everyone,
I need to work with graphene nanoribbon (AGNR and ZGNR) and I was advices to optimize the primitive structure, find the adecuate K-points for it and then expand the nanoribbon.
Unfortunately, I do not know how to do this process and I think there should be a rule of thumb or papers that can allow me to get those point in an easier way. Can someone help me with this???
Thanks in advance for your help,
Luis
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I wonder if there is any thumb-rule as such.../ Alignment of K-points in Monkhorst-Pack Grid is used for getting an accurate alignment to the Fermi Level..../ So, It may seem like more no. of K-points will give you a more accurate result... which is eventually not true..../ As you mentioned your device structures are mainly GNRs....so, It can be considered as the quasi one-dimensional system...Thereby once you make a device ("device from bulk") of such materials.... the Z-axis, that is the transport direction, should be the matter of concern...Thus, you can set a large K value in c-direction...and rest are not that significant....Therefore, you can take 1*1*99 or so, depending on the length of your device(no. of atoms actually), the method that are you using, and the computational complexity. ...../ But, as I said, a whole lot of K-point is not a better choice always...for e.g. to find out the band diagram of a unit cell of graphene if you use 1*12*12 (Ka-Kb-Kc), or 1*13*13 your results will be significantly worse than the result you will get for 1*3*3 (considering almost all other values up to 1*20*20). But, exceptionally, 1*16*16, or, 1*9*9 provides very good result.
So,setting the proper alignment of K-points, itself is tricky thing.... and as I said, depending on the required accuracy, method that you are using and obviously the dimensions of your system, ....you have choose it very carefully..... :)
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For further information.... You can visit this page.....
http://quantumwise.com/publications/tutorials/mini-tutorials/214-why-are-so-many-k-points-needed-in-the-transport-direction-in-a-device-calculation
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Dr. Blom
Sir, this will be very much helpful...if you kindly share your thoughts regarding this issue... :) / (Is there any "rule of thumb"??)
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For a 1D system like a nanoribbon it's very simple: use (1,1,many), where "many" is probably 50-100 or so. Alternatively a lower number can be used as long as it's a multiple of 3, like 27 or 30, but a safe number like 60 should not be much slower.
Graphite/graphene and derived structures can be a bit tricky, if you consider e.g. a whole sheet, since the electron density is around the K point, so you have to hit that by using preferably 3N points where N is integer.
In general the rule is that you start small and then increase until your results don't change a lot. You do not, however, have to do this for the "end result", like run the whole geometry optimization over and over for more and more k-points. You check some characteristic feature of the system, like the electron density, total energy, band gap, or so, for a given structure (fixed geometry which is at least reasonably close to the expected one), and then use the so-determined k-point sampling for the main calculation.
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Anders Blom
Thank you sir...for those details.....