1

##### General Questions and Answers / Re: How to extract real space hamiltonian from quantumatk

« Last post by**rounoknaimniloy**on

*January 27, 2023, 11:36*»

From low level entity I can get H(k) & S(k). But not real space hamiltomian? .

1

From low level entity I can get H(k) & S(k). But not real space hamiltomian? .

2

It's probably too long a period of time since there was any response to your post, but I thought I would give it a try. According to the previous forum post you are referencing, the selection of the "smallest atoms possible" is referring to the interface builder window with all calculated in-plane strains depending on whether the first layer, second layer, or both layers are strained. You want to select the dot on the lower panel that gives the lowest number of atoms while still minimizing the amount of in-plane strain resulting between the first and second layers.

The "repeat a bit on both sides of the interface" part would mean to add additional atomic layers to both sides of the interface using the "+" buttons you see under each displayed bulk material within the interface builder pulldown menu. Additional layers for each bulk material that are joined together to form the interface will be included in the supercell slab model. There are other ways to add more layers to both sides of the interface, but I will leave that out of the discussion for now. I hope that helps some.

The "repeat a bit on both sides of the interface" part would mean to add additional atomic layers to both sides of the interface using the "+" buttons you see under each displayed bulk material within the interface builder pulldown menu. Additional layers for each bulk material that are joined together to form the interface will be included in the supercell slab model. There are other ways to add more layers to both sides of the interface, but I will leave that out of the discussion for now. I hope that helps some.

3

No, the two calculations are not really the same numerically. Physically yes, kind of, but the algorithms are very different. Just to start with, there is no diagonalization of the Hamiltonian in the NEGF method, and also there are contributions to the Hamiltonian (and thus energy) from the electrodes, so one cannot even say the number of atoms is strictly the same. I honestly don't think you will find much benefit from this analysis unless you plan to write your own NEGF code...

The details of the NEGF method we use are well documented and links can be found in the manual, at the top of https://docs.quantumatk.com/manual/NEGFDevice.html, plus https://doi.org/10.1088/1361-648X/ab4007.

The details of the NEGF method we use are well documented and links can be found in the manual, at the top of https://docs.quantumatk.com/manual/NEGFDevice.html, plus https://doi.org/10.1088/1361-648X/ab4007.

4

The difference of 15 eV is of the total energy, not per atom. The number of atoms are the same for periodic and Green's function calculations. The reason I want to do this comparison is curiosity. I am learning the Green's function method, and I am wondering on what extent does the Green's function method is equivalent to periodic calculations.

In principle, these two calculations shall be equivalent, so the difference in total energy must result from the interface between leads and the scattering region. So, could you please provide more details on how QuantumATK dealing with the lead/scattering region interface?

Best,

Yun-Peng

In principle, these two calculations shall be equivalent, so the difference in total energy must result from the interface between leads and the scattering region. So, could you please provide more details on how QuantumATK dealing with the lead/scattering region interface?

Best,

Yun-Peng

5

The EDP has a very high resolution, what you are seeing are variations across individual atoms. It will become flat asymptotically towards the electrodes, not generally flat.

6

Is that 15 eV per atom or in total? Same number of atoms? Also the algorithm is quite different, so since energies are not absolute, comparing two different calculations like this does not really make sense. And anyway, why do you want to compare them?

7

I understand that the Green's function method does not have a "total energy" equivalent to periodic calculations due to the charge transfer between leads and device. However, if I simulate a crystal using Green's function method, that is, setting the lead and the device to be the same material, there shall be no charge transfer at all; therefore the total energy from Green's function calculations shall be the same with periodic calculations. I use hexagonal BN as an example, however the results show a difference of 15.5 eV. I cannot understand why. Please give suggestions. Thank you very much!

Yun-Peng

Yun-Peng

8

The numerical noise should not be the cause of the negative modes.

If you obtain negative modes at larger supercells it indicates that the supercell size was not converged yet.

One can "accidentally" have fairly good phonon bands in smaller supercells if interactions are cut off in a manner that fulfills or almost fulfills the lattice symmetry.

Tue

If you obtain negative modes at larger supercells it indicates that the supercell size was not converged yet.

One can "accidentally" have fairly good phonon bands in smaller supercells if interactions are cut off in a manner that fulfills or almost fulfills the lattice symmetry.

Tue

9

Thank you for your answer!

10

Thank you so much for your detailed response.

I was interested for integrated DOS over energy - I successfully did that.

Need you expert view point for my system.

I have six width long nanoribbons long C direction as shown in figure. I just pick it randomly but I want to know what kind of analysis i should do to verify that this 6 width long nanoribbons is fine - similarly how I can do analysis that these 4 layers are also fine?

I haven't figure it out how long the nanoribbons should be? how many layers are needed?

Need your expert point of view on it.

Thank you

I was interested for integrated DOS over energy - I successfully did that.

Need you expert view point for my system.

I have six width long nanoribbons long C direction as shown in figure. I just pick it randomly but I want to know what kind of analysis i should do to verify that this 6 width long nanoribbons is fine - similarly how I can do analysis that these 4 layers are also fine?

I haven't figure it out how long the nanoribbons should be? how many layers are needed?

Need your expert point of view on it.

Thank you