Messages

1081

General Questions and Answers / Re: how to extract .nc file after bsse correction?

« on: April 10, 2017, 15:20 »

I think you should be able to get the total energy calculated automatically if you add 'TotalEnergy' as an analysis object in the Scripter, as described in the tutorial you have mentioned http://www.quantumwise.com/documents/tutorials/latest/Grimme/index.html/chap.graphene.html.   

1082

General Questions and Answers / Re: why the equivalent bulk density matrix are still used for neutralAtom method

« on: April 10, 2017, 15:13 »

I guess there was a mistake in the description of the NeutralAtom class in the old version of the reference manual. Please have a look at our up-to-date, online reference manual at quantumdocs: http://docs.quantumwise.com/manuals/Types/NeutralAtom/NeutralAtom.html and http://docs.quantumwise.com/manuals/Types/EquivalentBulk/EquivalentBulk.html.

1083

General Questions and Answers / Re: Why am I getting a small bandgap in density of states of graphene?

« on: April 10, 2017, 11:27 »

I meant using the 27x27 k-point mesh for self-consistent calculation, not DOS. But as you may see in the figure enclosed, the 27x27 grid is not that bad, at least it does not open the energy gap in the graphene's DOS.

Enclosed you can find graphene's DOS calculated in ATK-2016.4, using the 129x129 k-point mesh for DOS calculations. It looks perfectly fine.

So, I cannot say right away if that is a bug in ATK-2014.3 version. If so, it has certainly been fixed in newer versions of ATK. 

1084

General Questions and Answers / Re: Why does carrier density in GFET and GFET(reduced the gate) is the same?

« on: April 10, 2017, 11:00 »

Could you enclose the scripts and log files related to these calculations?

1085

General Questions and Answers / Re: Energy Bands vs Distance

« on: April 10, 2017, 09:50 »

You may do it using the projected local density of states (PLDOS) analysis as shown in the following tutorial http://docs.quantumwise.com/casestudies/ag_si_interface/ag_si_interface.html.

1086

General Questions and Answers / Re: optimization

« on: April 10, 2017, 09:45 »

Getting no negative frequencies do not guarantee that the device structure is the most stable one, i.e., the total energy minimum found corresponds to the global minimum. In general, it only tells you that the structure is not at the saddle point, i.e., far from a phase transition.

Note that negative frequencies may also be an artifact of the calculation, e.g., using improper pseudopotentials can give rise to an inaccurate calculation of forces acting on atoms in the system.       

1087

General Questions and Answers / Re: Molecular electronics optimization.

« on: April 10, 2017, 09:28 »

If you have a convergence problem for higher voltage calculations, you may do IV calculations by increasing bias voltage in small steps (it should perhaps be smaller than 0.5 V adopted in your calculations), using the lower bias converged calculation as the initial state for the higher bias calculation. Take a look at the section on "Finite-bias calculations" in the ATK transport tutorial http://docs.quantumwise.com/tutorials/atk_transport_calculations/atk_transport_calculations.html.

Note that for some systems applying high voltages does not make sense, as the system may not stand high currents; this is something to figure out by consulting the corresponding experimental data in the literature. 

1088

General Questions and Answers / Re: Why am I getting a small bandgap in density of states of graphene?

« on: April 7, 2017, 16:02 »

What version of ATK are you using? I guess the problem is that your k-mesh does not include K-symmetry point, and that is crucial to avoid an artificial band gap opening (due to the use of relatively coarse grid) in the graphene's band structure.

Having the 288x288 k-point grid does not guarantee that the K-point is a grid point. In the ATK-2016.4 (as well as in the VASP code adopted for the calculations in the paper mentioned), there is an option "Shift to G-point", in this case the the 288x288 k-point grid would include K-symmetry point indeed. 

If you have an old version of ATK, you may adopt an odd k-point grid that includes not only Gamma-point, but also the K-point, e.g., the 27x27 (129x129) k-point grid for self-consistent (DOS) calculation. It worked for me.

1089

General Questions and Answers / Re: Calculation Time

« on: April 6, 2017, 13:48 »

Given a small difference in the number of atoms as well as in the number of k-points for these two cases, it might be faster to actually do these two calculations than to guess which of the two is the fastest.

1090

General Questions and Answers / Re: How to build a MTJ device with different oxide 'stack'?

« on: April 6, 2017, 10:56 »

The MTJ plugin in the Builder allows you building a specific junction type (metal-insulator-metal) indeed.

Building a general structure can be done using other Builder tools. In principle, you can build any structure in the Builder. For example, you may build a multilayer oxide structure using the Interface plugin by adding oxide layers in a step-by-step manner, see http://docs.quantumwise.com/tutorials/ag_au_interface/ag_au_interface.html for how to use the Interface plugin, and then match the oxide heterostructure to the Fe electrodes, using the same Interface plugin.

Note that you need to consider optimizing the separation distance between the neighboring oxide layers, as well as between the oxide stack and metal electrode. This can be done in a set of separate calculations for individual interfaces, and then building the entire structure of the MTJ device. Afterwards, you may still do additional geometry optimization of the entire central part of the device, if needed.

1091

Installation and License Questions / Re: Why does VNL requires me to configure the license everytime I start it.

« on: April 6, 2017, 10:22 »

Regarding license issues, please contact QuantumWise support: support@quantumwise.com.

1092

General Questions and Answers / Re: Lattice thermal conductivity

« on: April 5, 2017, 23:00 »

Regarding #1, why would you need to use millions of atoms to calculate the thermal conductivity of a crystalline material? I guess you may do several calculations for much smaller supercells to see how fast the conductivity converges to a certain value.

1093

General Questions and Answers / Re: Wrong result as per theoretical formula

« on: April 5, 2017, 22:21 »

I am not sure I have understood your question. Have you calculated the elasticity tensor (elastic constants) in ATK? Do the obtained elastic constants agree with experiment?

1094

General Questions and Answers / Re: Molecular electronics optimization.

« on: April 4, 2017, 21:25 »

If you want to get consistently accurate results for the entire IV curve, you should converge the scf calculations for each bias voltage applied.

1095

General Questions and Answers / Re: I-V curves for 2D materials

« on: April 4, 2017, 17:06 »

- Adding metallic electrodes will make your calculation more realistic, but computationally more expensive. On the other hand, if you are just interested in seeing a difference between the electron transmission with and without a scatter (molecule), your setup might still be good enough for that purpose.

- VNL-ATK and TranSiesta are based on the same methodology. Using short electrode and electrode extension regions might give rise to convergence problems as well as to incorrect electron transmission. In the paper, they have one molecule in-between two electrodes, so the transmission should converge to a certain value upon increasing the size of the left and right electrode extensions.  If they used too short electrode extensions, the result could be wrong.

Did you mean the density of molecules defined with respect to lateral direction (perpendicular to the transport direction)? Otherwise, the density of molecules does not depend on the size of the central region.

In the paper, I do not see any strong effect of the molecule adsorption on the current, at least if I am looking at Figure 4.

I guess the idea is not to induce the discrepancy, but to figure out what happens in reality, meaning that you should build a reliable (but still simple enough)  model that reflects experimental conditions.