Messages

1

General Questions and Answers / Difference Between electrode_extension_lengths and equivalent_electrode_lengths

« on: November 21, 2025, 00:06 »

Hi all,

Could anyone clarify the physical meaning of electrode_extension_lengths in the device configuration, and how it influences the calculation results? The manual only states that it is “the desired equivalent electrode extension length of each electrode,” which is not very informative.

In a related forum discussion (https://forum.quantumatk.com/index.php?topic=10735.0), it was mentioned that the calculation results depend on this parameter. What is the underlying theory for this dependence?

We already have the parameter equivalent_electrode_lengths, which defines how much of the central region should be equivalent to the electrodes which is clear. But how is electrode_extension_lengths fundamentally different?

In older versions of the manual and ATK (e.g., https://docs.quantumatk.com/tutorials/atk_transport_calculations/atk_transport_calculations.html), this parameter did not exist.

2

General Questions and Answers / Unexpected Transmission Channel in Monolayer Graphene DFT Transport CalculationD

« on: October 10, 2025, 22:44 »

Dear QuantumATK team,

My research strongly relies on ATK for calculating transport properties, and the LCAO framework in ATK has been extremely helpful and valuable for my studies.

Recently, I calculated the transmission spectrum of monolayer graphene. Since graphene is a well-studied material with a simple structure, it is often used as a benchmark system. However, I encountered a strange problem.


(Web image here. If it's not shown, find in the attachment.)

As shown in the figures:
- Fig.(a) presents the graphene band structure calculated using VASP along a chosen direction c. b direction has vacuum layer. The lattice has been orthogonalized, showing the well-known two Dirac cones, with the Fermi energy set to zero. ATK band-structure calculation matches this result.
- Fig.(b) and (c) show the bulk (unit cell) non-bias transmission spectrum calculated using ATK 2024. In (b), the x-axis is k-parallel and the y-axis is energy; in (c), the x-axis is energy and the y-axis is transmission. Overall, (b) looks very similar to the band structure in (a) which right, but there is one abnormal transmission channel (green line). This green channel causes the transmission at the Fermi level in (c) to be 1 instead of 0, which is clearly incorrect. The calculation was done using DFT with Dojo pseudopotentials.

To identify the issue, I tried several approaches: Using ATK 2022 instead of 2024; Rewriting the structure with cleaner fractional coordinates (Fig.(d)); Increasing the k-grid to up to 18×1×96 (Fig.(e)). None of these changes fixed the problem.

I also tried using SG15 pseudopotentials (Fig.(f)), it’s even worse that two fake conduction channels appeared (green lines).

I also tried building a bulk-like device by repeating the unit cell several times (Fig.(g)). In that case, the fake channel disappeared, but the transmission became non-integer, and the non-zero region below the Fermi level appeared as discrete lines rather than continuous bands.

Finally, I tested the Slater–Koster (method following the official tutorial (https://docs.quantumatk.com/tutorials/transmission_gr_mos2/transmission_gr_mos2.html). The result was correct and free of fake channels. However, since my main workflow is based on DFT, the S-K method is not suitable for my project. The fact that the S-K method works correctly suggests that my structure setup is fine and the problem might come from the DFT implementation in ATK.

I have attached the calculation files for both the bulk (unit cell) case (corresponds to Fig.(e)) and the bulk-like device case (corresponds to Fig.(g)).

I sincerely hope to receive your advice.

Thank you very much!
Kai

3

General Questions and Answers / Definition of Dzyaloshinskii–Moriya Interactions in QuantumATK

« on: May 10, 2025, 00:35 »

Dear ATK developers,

In recent versions of QuantumATK, the implementation of Heisenberg exchange and Dzyaloshinskii–Moriya interaction (DMI) using the Green's function method has been a valuable tool for us, improving computational efficiency greatly compared to traditional methods.

While reviewing the documentation for the Heisenberg interaction, https://docs.quantumatk.com/manual/Types/HeisenbergExchange/HeisenbergExchange.html, I found the Hamiltonian of Heisenberg exchange is H = - summation of  J*Si*Sj for i not equal to j, which is clear, and it implies the J will be double counted for ij and ji, with positive J favoring ferromagnetic alignment. However, I couldn’t find the corresponding Hamiltonian form for the DMI in the documentation. Could you please clarify the exact definition of the DMI term used in the output?

Additionally, for the Heisenberg interaction, the output clearly distinguishes between values with and without spin scaling, as shown by the columns:
|  Distance   Local index   Translation    Symbol     J_ij      J_ij/(S_i*S_j) |
This helps us interpret the results appropriately. But in the DMI output, we see only:
|  Distance  Local index  Translation   Symbol   |D|       (Dx,  Dy,   Dz)     |
There is no mention of spin scaling. Could you confirm whether the reported |D| values include the product Si*Sj, or are they independent of spin magnitude?

Best regards,
Kai Huang

4

General Questions and Answers / Inconsistant transmission between bulk and bulk-like device

« on: June 2, 2024, 21:04 »

I'm calculating the transmission of a 2D material.

I tried two approaches:

1. Transmission of the bulk in the unit cell.
2. Transmission of the bulk-like device, where the left electrode, right electrode, and central area are all the same unit cell structure.

I assumed the transmission results would be identical. However, the results are different:

[image in the attachment]

(image link in case it doesn't show: https://imgur.com/wRPUPpw)

The left image shows the unit cell transmission, while the right image shows the bulk-like device transmission. Both calculations use GGA with polar spin and no SOC included.

You can observe that the transmission curves relative to the Fermi energy are different, with the transmission from the unit cell being larger.

My question is, why could this difference occur? Should the results be the same, and which one should be considered correct?

Thanks,
Kai

5

General Questions and Answers / Half integer transmission in hdf5 file

« on: April 21, 2024, 06:55 »

Hello,

I conducted the transmission calculation for a bulk system and attempted to plot the transmission concerning k by parsing the hdf5 file. In this instance, I initiated the DFT calculation, incorporating Spin-Orbit Coupling and noncollinear magnetic moments. GGA method and SG15-SO pseudopotential. Subsequently, the transmission was calculated. Within the output hdf5 file, I observed that the transmission is delineated in the data tree as follows:

- TransmissionSpectrum_0
    - transmission
        - 0
            - data
        - 1
            - data
        - 2
            - data
        - 3
            - data

For each 'data,' there is a 2D array. Two indices represent k points and energy, respectively.

However, two questions arise:

1. Is it correct that 0, 1, 2, 3 above represent the transmission of spin all, x, y, and z, respectively? Or do they represent spin all, y, z, and x? Or something else?
2. In the transmission->0->data, I noticed that the transmission plot matches the electron band structure well, which implies the validity of the result. However, I saw many half-integer transmissions, like 0.5, 1.5, 2.5, etc. It seems that each band only contributes 0.5 transmission channel rather than 1. So, I would like to inquire about the meaning of this dataset. Can I multiply it by 2 to get the right result?

Additionally, I attempted the transmission calculation without SOC, i.e., choosing just spin polarized. The transmission output in the hdf5 file is as follows:

- TransmissionSpectrum_0
    - transmission
        -0
            - data

This time, there's no spin projection. But again, each band contributes only 1 transmission channel rather than 2 (for the two spin eigenstates). Again, what's the meaning of this data in hdf5, and can I just multiply by 2?

Thanks,
Kai