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Messages - Tue Gunst

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1
Hi Alireza,
Thanks for the detailed comments.
I see that the flexibility in controlling which solver to be used is not present in Bandstructure currently.
We are currently implementing/exposing iterative solvers in Bandstructure - but you will first be able to use that in the next release.
In the next release, it should be possible to perform bandstructure simulations on millions of atoms with iterative solvers (avoiding calculating all the valance bands).
For the present release, the only solution is to optimize for memory and reduce the number of k-points.
Tue

2
General Questions and Answers / Re: ThermoChemistry Analyzer
« on: February 15, 2023, 14:57 »
There are two use cases of the multiplier:
1) To balance the chemical reaction. There should be a balance in the chemical reaction to secure that the number of each element is the same in the reaction.
Notice that you can click the yellow "auto balance" warning in the products overview to let the GUI balance the reaction.
2) To compare similar reactions with different numbers of equivalent/similar reactants or products.
For the purpose of consistently comparing several reactions, it is necessary to put the reaction enthalpy on an equal footing. Therefore it might be necessary to scale reactions to be comparable (on the same energy scale).
Trivial example: doing the same simulations with twice the number of the same reactants and products one needs to scale by a factor of 2 to get the same result.

For the gas analysis: there are dedicated workflows in the workflow builder to make sure that you perform the calculations correctly for either surfaces or gas elements.
There are different classes (CrystalThermoChemistry and GasThermoChemistry) for to be used for either crystals or liquids.

For the pressure and temperature: In the analyzer, you can modify the partial pressure for each element separately (it is a simple click on each product/reactant). Everythink in the analyzer is visualized as a function of the temperature.

A tutorial in using the analyzer can be found here:
https://docs.quantumatk.com/tutorials/thermochemistry_analyzer/thermochemistry_analyzer.html

Hope that clarified your questions,
Tue

3
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

4
I recommend to not set automatic repetitions, use_wigner_seitz_scheme=True, and not set a max_interaction_range.
If the max_interaction_range is lower than the range of physical coupling, the crystal symmetry is broken which probably results in negative phonons.
In that case, the wigner_seitz_scheme cannot restore it (hence it is disabled by default in the scriptor but can be set to true still in the script if needed).
Instead, use_wigner_seitz_scheme=True and converge the supercell size until physical interactions are included (converged dispersion and no negative modes).
As shown by the example, 7 repetitions seem to be sufficient with the wigner_seitz_scheme.

5
Attaching a quick check with LCAO settings. Takes a few minutes and reproduces the result from the paper well.
I am running the PW calculation but that will take some hours.
One thing that I notice is that you have set use_wigner_seitz_scheme=False.
This is non-default and not recommended since you will need very large supercell if not allowing the symmetry conserving copying of periodic data in the supercell.
Tue

6
Hi,
The hamiltonian can be accessed in orbital space.
Please refer to the LowLevelEntities:
https://docs.quantumatk.com/tutorials/low_level_entities/low_level_entities.html
Real-space eigenstates can be obtained from the dedicated analysis objects (see Eigenstate or Blochstate objects in the reference manual).

7
Hi,
The Fermi-broadening does have a significant impact on DFT results.
Specifically, it impacts how fast scf converges and the final state (if the broadening is too large).
For a semiconductor I would reduce the smearing to 100-300 K. This means that more k-points are needed.
For a metal a larger broadening can be used and lower k-points sampling.
In any case, the broadening should be reduced and k-point sampling increased until simulations are converged.
In general the defaults are good but you seem to use a much larger broadening.

8
Hi Krabidix,
Did you use a force-field or DFT? Also I assume that you optimized the structure and used the same 7 × 7 supercell?
Tue

9
An optimization usually refers to adjusting (minimizing) the total energy by moving atoms.
Using the lattice constraint one does this for fixed lattice (existing unitcell whatever strain is present).
If you also want to adjust the unitcell by varying the strain while minimizing the total energy, then you don't need the lattice constraint.

10
The KpointDensity is first introduced in 2020 but you can use the older conventional MonkHorstPack sampling for k-points.
If you follow the scripter workflow (add SiC from database + add PlaneWaveCalculator +  add BaderCharge) the script generated will be consistent with the version that you currently use.

11
General Questions and Answers / Re: Raman spectra calculation
« on: September 16, 2021, 17:29 »
I assume that the mechanism that you refer to is the peak position shift in Raman peaks?
The Raman peaks will shifts position due to anharmonic effects with temperature.
The best option currently would be to perform MD for a supercell to include anharmonic interaction.
In general, ensemble averaging the Raman spectra should do the job but that will of cause be computational heavy for a supercell.
A better solution would be to calculate anharmonic shifts in the phonon frequencies directly. QuantumATK currently doesn't support that but it is in the pipeline so stay tuned.

12
General Questions and Answers / Re: CNT Schottky diode
« on: August 12, 2021, 09:42 »
In the Builder->interface builder:
When you have added the two structures there is the option to click "Shift Surfaces" before clicking 'Create'.
If a valid force-field is available the "Shift Surfaces" menu can suggest a displacement (Click calculate displacement).
If not then one needs to manually substract a z-displacement to select a sensible interface distance (which can then be optimized separately if not known from experimental data etc).

13
Hi,
The gate voltage in the just the actual value of the potential in the metallic regions applied.
In the double gate setup you apply the same value to both metallic regions. Having double gates in 2D or gate-all-around in 1D makes gating more effective.
An example of the double gate configuration can be found here:
https://docs.quantumatk.com/tutorials/inas_p-i-n_junction/inas_p-i-n_junction.html

14
General Questions and Answers / Re: second harmonic generation
« on: August 10, 2021, 09:47 »
Not all materials are nonlinear optical materials. SHG is zero in systems with inversion symmetry for instance.
I recommend checking the GaAs example for instance on the object to learn about typical parameter settings:
https://docs.quantumatk.com/manual/Types/SecondHarmonicsGenerationSusceptibility/SecondHarmonicsGenerationSusceptibility.html

15
General Questions and Answers / Re: Raman Spectra
« on: August 3, 2021, 10:59 »
The momentum matrix elements is available as a low level entity in QuantumATK and can be accesses in the following way:
"
p_matrix = calculateMomentumMatrixElements(
        configuration,
        kpoint=None,
        number_of_states=None):
"
The function calculates the momentum matrix elements for a bulk configuration at a specified k-point (fractional).
The x-component of the momentum matrix elements is defined as
p_nm(k) = <mk | -i * hbar * d/dx | nk>,
where |nk> is a Bloch state in band 'n' at the k-point 'k', and likewise for the y- and z-components.
For Unpolarized, Noncolliner and Spin-orbit calculations, each matrix has a shape (3, *N*, *N*), where *N* is the number of bands included.
For Polarized calculations, the shape is (2, 3, *N*, *N*) with the first index corresponding to Spin.Up and Spin.Down.

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