Messages

1021

General Questions and Answers / Re: The problem of device NEGF DFT convergence?

« on: June 28, 2017, 11:19 »

The structure looks OK. Which version of ATK are you using? In ATK-2017.b1, there is an option of using a more robust and faster Poisson solver:  [Parallel] Conjugate gradient.

In the Device Algorithm settings, you may also consider using Neutral Atom instead of Equivalent Bulk for initial density.

I would not change the Contour Integral Settings for the moment, unless you are sure that there are deeply-lying states of Ni. In this case, you might need to increase the value of integral_lower_bound. This can be checked by band structure calculation for bulk Ni.

1022

General Questions and Answers / Re: electron-phonon interaction in two probe system

« on: June 28, 2017, 10:09 »

You may see one example of how the thermoelectrics calculations can be done in ATK in the following tutorials at http://docs.quantumwise.com/tutorials/thermoelectrics_cnt_isotope/thermoelectrics_cnt_isotope.html and http://docs.quantumwise.com/tutorials/phonon_calcs/phonon_calcs.html.

The calculations are done in the linear response regime, and no T_c and T_h are explicitly introduced in the ATK calculations. To compute the thermoelectric coefficients and ZT in a post-processing manner, one can use a thermoelectrics plugin as discussed in the tutorials, and the temperature T=(T_c+T_h)/2 is to be manually set in the widget. 

In ATK, you may also calculate interfacial thermal conductance, using a molecular dynamics (MD) approach  as described in http://docs.quantumwise.com/tutorials/interfacial_thermal_conductance/interfacial_thermal_conductance.html. In this case, the temperature gradient across the interface is explicitly taken into account in the non-equlibrium MD simulations.   

1023

General Questions and Answers / Re: electron-phonon interaction in two probe system

« on: June 27, 2017, 14:52 »

I am not sure I understand your problem. The T temperature is for ZT-calculation only; ZT is calculated in a post-processing manner after all the physical quantities in the ZT formula have been computed. What are the actual ATK calculations you are referring to?

1024

General Questions and Answers / Re: The problem of device NEGF DFT convergence?

« on: June 27, 2017, 10:47 »

To have an idea on the convergence issue, it would be helpful to see the actual structure of the junction that is missing in the scripts.

1025

General Questions and Answers / Re: electron-phonon interaction in two probe system

« on: June 27, 2017, 10:42 »

The temperature in the formula for ZT is given as an arithmetical average, T=(T_H + T_C)/2.

1026

General Questions and Answers / Re: Questions about Geometry Optimization of Defective Graphene with Al atoms

« on: June 27, 2017, 10:39 »

The target structure seems to have a single phosphorous atom (i.e., not multiple Al atoms) as a point defect acting as dopant. So, you have no target structure.

I do not know if Al atoms can go substitutional in the graphene lattice. The Al atoms might rather prefer to be on top of the graphene sheet.  In any way, you may also consider using other force fields available in ATK-2016.4 or 2017.b1 to study this problem. 

1027

General Questions and Answers / Re: Calculating HOMO-LUMO energies at different bias voltages in molecular junction

« on: June 26, 2017, 14:08 »

I think you may still try doing the Molecular Projected Self-Consistent Hamiltonian (MPSH) analysis (described in the tutorial) for the molecular junction at nonzero bias voltages. Calculating Projected Device Density of States would complement this analysis, as you might also be able to see some PDDOS peaks related to molecular levels.

1028

General Questions and Answers / Re: Questions about Geometry Optimization of Defective Graphene with Al atoms

« on: June 26, 2017, 13:03 »

Could you give more computational details of how exactly you have been doing these calculations? It might be helpful if you enclose a python script related to geometry optimization of the structure.

1029

General Questions and Answers / Re: Calculating HOMO-LUMO energies at different bias voltages in molecular junction

« on: June 26, 2017, 13:00 »

I think the best way would be to calculate the transmission spectrum at different bias voltages, see the following tutorial http://docs.quantumwise.com/tutorials/molecular_device/molecular_device.html. The molecular energy levels correspond to electron transmission peaks.

I would like to notice that if the coupling between the molecule and electrodes is sufficiently strong, then there might exist a significant broadening of the molecular levels, i.e., the energy spectrum is no longer discrete as in the case of an isolated molecule. 

1030

General Questions and Answers / Re: Too much expansion of the unit cell : ReTe2

« on: June 25, 2017, 11:45 »

Assuming that you are using periodic boundary conditions in the out-of-plane direction, you have to make sure that the vacuum thickness is sufficient to avoid a spurious interaction between periodic images of your molecule-substrate structure.

An alternative option is to adopt non-periodic boundary conditions by selecting the Poisson solver type to FFT2D and setting the Left(C) (Right (C)) boundary condition to Dirichlet (Neumann) type. It will not be as fast as using FFT3D, but might be a more accurate approach to your problem. You may give it a try for both options to see if there is any difference in the results obtained.

1031

General Questions and Answers / Re: Strange Band-structure of ZrSe2

« on: June 25, 2017, 11:33 »

Is there any particular reason that you have adopted a triclinic unit cell for ZrSe2? Have you optimized your structure?

Another thing is that using the GGA or LDA density functional might not allow you to calculate the band gap with the required accuracy, see J. Phys. Chem. C 119, 13169 (2015).

An additional note, I do not think you need that many k-points (71x71x1) to sample the Brillouin zone for self-consistent calculations. Also you may try using a more accurate pseudopotential/basis set such as SG15/Medium.

1032

General Questions and Answers / Re: Can I run this command using VNL?

« on: June 25, 2017, 10:59 »

what's the meaning of the difference in the EDP or HDP curve between the left and right electrodes, just representing the band bending near the interface?

The macroscopically-averaged HDP shows the band bending far from the interface, and it reaches constant values in the left and right electrodes. Note that band bending is not defined at the interface, as its definition relies on the Thomas-Fermi approximation that is not valid at or sometime even near the interface.

One may use the HDP curve together with the PLDOS graph to understand band alignment between two dissimilar semiconductor structures.

1033

General Questions and Answers / Re: Too much expansion of the unit cell : ReTe2

« on: June 23, 2017, 14:00 »

I would guess that you may want to constrain the Bravais lattice type, provided that you have built the unit cell corresponding to the ReTe2 Bravais lattice of your interest, i.e., assuming that you are not looking for a different phase of this material.

What is definitely better to do for a monolayer geometry optimization is to constrain the Bravais lattice vector in the out-out-plane (z) direction. In this case, the vacuum thickness would not change, and you may avoid nonphysical geometry optimization along the z-direction.

1034

General Questions and Answers / Re: Some thing wrong with MoTe2 calculations ?!!!!!!!!

« on: June 23, 2017, 10:29 »

Three things:

- You do not need to use that many k-points to sample the 2D Brillouin zone in the self-consistent calculation of bulk MoTe2. So, you may consider reducing it from 41x41x1 to, e.g., 12x12x1 Gamma-centered k-point grid. 

- I would suggest changing the pseudopotential/basis set to SG15-type/Medium basis set.

- In principle, you do not need to set the same ATK-DFT calculator to do the band structure analysis after geometry optimization.

If you make the changes mentioned, you must be able to do this calculation rather quickly, at least it worked for me, taking 6 minutes on my laptop.   

   

1035

General Questions and Answers / Re: can I do tensile testing of a unit cell ?

« on: June 23, 2017, 09:50 »

You may have a look at the following tutorial, http://docs.quantumwise.com/tutorials/uniaxial_biaxial_stress/uniaxial_biaxial_stress.html.