### Author Topic: Number of K-point for transmission spectrum  (Read 188 times)

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#### NW

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##### Number of K-point for transmission spectrum
« on: March 25, 2021, 17:07 »
Hi,

I need to know how to choose the correct number of k-point (kA and kB) for calculation of transmission spectrum.

Thanks
« Last Edit: March 25, 2021, 20:10 by NW »

#### mlee

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##### Re: Number of K-point for transmission spectrum
« Reply #1 on: March 29, 2021, 14:50 »
It is hard to suggest the correct number of K-point. It depends on the model. We guide the preset densities.

#### Anders Blom

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##### Re: Number of K-point for transmission spectrum
« Reply #2 on: April 14, 2021, 09:03 »
Or put differently: the only way to know is to keep increasing the number of k-points until the transmission spectrum (or better yet, the current) doesn't change.

Note that in general you need more k-points than for the SCF loop because the transmission coefficients as function of k (and, sometimes, energy too) can be quite spiky, whereas the density is more smooth and converges faster (excepts for some tricky systems, of course). Cf. the discussion here: https://docs.quantumatk.com/tutorials/nisi2-si/nisi2-si.html#transmission-spectrum-convergence

#### NW

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##### Re: Number of K-point for transmission spectrum
« Reply #3 on: April 14, 2021, 23:49 »
Dear Anders,

Thanks for your explanation. The transmission spectral of my system is so sensitive to the number of k-points and when I increased the number from 7 to 50 by choosing random numbers in between I got completely different results.

#### Anders Blom

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##### Re: Number of K-point for transmission spectrum
« Reply #4 on: April 15, 2021, 00:30 »
For the current? Well, yes, I guess it goes to show the importance of converging in this number, a point perhaps not made strongly enough in our tutorials or manuals.

The underlying reason is basically that the transmission is a resonance, so if the electrode band structure has several local minima/maxima in the energy range, then at any given bias the overlap between left/right states is almost random. Notably, you don't have to converge the whole transmission though, just the relevant range which is intergrated for the current. Often the band structure and hence transmission is more complex farther from the Fermi level, but this makes no difference for the current at low bias at least.

#### NW

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##### Re: Number of K-point for transmission spectrum
« Reply #5 on: April 15, 2021, 10:22 »
Dear Anders,

Is there any way that I can send my script to you?

Thanks for considering my request.

#### Anders Blom

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##### Re: Number of K-point for transmission spectrum
« Reply #6 on: April 16, 2021, 01:16 »
You can email via the Forum, my profile is accurate.

#### NW

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##### Re: Number of K-point for transmission spectrum
« Reply #7 on: April 17, 2021, 00:08 »
Hi,

I tried to send a message via forum but it was not possible. It seems that it has been blocked.

#### Anders Blom

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##### Re: Number of K-point for transmission spectrum
« Reply #8 on: April 17, 2021, 02:06 »
Aha, that might be, but isn't my email visible when you try that? I can see yours :-)