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Messages - kstokbro

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16
General Questions and Answers / Re: Spill-in terms in negf
« on: April 15, 2015, 23:18 »
We have not published this yet. ATK use an LCAO description, and the challenge is that many basis orbitals in a device have tails outside the central region, or basis orbitals in the electrode has tails inside the central region. All these tails must be included correctly in the DFT implementation, and this is what we call spill in.

ATK is one of the only codes where this is done properly, and this is why we can handle very efficiently heterogenous systems, like the interface between Ni-graphene or metal-organic interface

K. Stokbro, M. Engelund and A. Blom, Atomic-scale model for the contact resistance of the nickel-graphene interface, Phys. Rev. B 85, 165442 (2012).

K. Stokbro and S. Smidstrup, Electron transport across a metal-organic interface: Simulations using nonequilibrium Green's function and density functional theory,  Phys. Rev. B 88, 075317 (2013).




17
For molecules you can try increasing the temperature to get better SCF convergence. In ATK-2015 we have the LBFGS which converges faster, and we have also optimized the force calculation. For molecules you may use the FFT solver for the electrostatics, it is much faster and is default in ATK-2015. ATK should be the fastest code available when you have more than 200 atoms in your system.

19
General Questions and Answers / Re: Effect of gate metal
« on: April 7, 2015, 21:45 »
When your two electrodes are of different materials, they may have a different work functions, i..e their fermi levels are different. When you form the two probe, at zero bias the two fermi levels are alligned and it is now the two vacuum levels that are different. In the output file we report the value of the fermi levels, lets call that value Ef

Now you put in a  metallic region as gate. If you specify the gate potential to zero, it will correspond to modelling a gate material with work function W=-Ef with a applied gate potential zero. However, if you would like to model another gate material with work function W1, you actually need to apply a potential V  to simulate the situation of a zero gate potential V1=0.

Thus, the relation between the experimental gate potential V1, and the calculational gate potential V, is
V = V1+W1+Ef

Thus, if you have a calculation with Isd versus Vgate, you can transform it to a physical gate using the transformation
V1 = Vgate - (W1+Ef)

21
General Questions and Answers / Re: piezoelectric
« on: April 1, 2015, 11:48 »
Check:
http://www.quantumwise.com/documents/tutorials/latest/Polarization/index.html/

and:
http://www.quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/ref.piezoelectrictensor.html

Note that we are not currently including internal relaxations in the piezoelectrictensor, this will be part of ATK-2015

22
You have some sign error, possibly because you need to multiply mulliken population with -e to get the charge?

23
General Questions and Answers / Re: bond order
« on: April 1, 2015, 11:39 »
You must calculate a Mulliken population object and then ask the object for the mulliken population per bond (must be done in a script)

see
http://www.quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/ref.mullikenpopulation.html

24
In the gui you will need to save the script with script detail set to default. In this case you will get all the details out of the basis set. You can now change the details of the basis set in the script.

I attach the input files and scripts from the paper

25
Please check:
http://www.quantumwise.com/documents/tutorials/latest/Phonon/index.html/
however, it is related to the sign of the bias differen when you have a temperature difference between the two electrodes.

26
Check the tutorial:
https://docs.quantumatk.com/tutorials/opening_a_band_gap/opening_a_band_gap.html

i.e. build bilayer graphene armchair or zig-zag,
 change C into Si,
relax

Edit: Updated link

27
Thanks for noting this, we will look into it

29
Dear Fang,
In ATK there is a line minimization along the conjugate force direction. The step length is estimated from a parabolic interpolation, and it is our experience that this estimate is not reliable if the step length is too long, we therefore restrict the step lengths. The step length is specified as a physical quantity, and you can use any lenght scale, i.e. 2*Bohr or 1*Ang, 10**-10*Meter, etc.

In ATK-2015 we will release some additional relaxation methods

30
Which version of VNL are you using?
In the latest versions we have a color bar which relates the color to a value. Without the color bar and no other information about your system and VNL setup it is difficult to answer your question.

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