Topics

1

General Questions and Answers / Purpose of Electrode Extension ???

« on: January 10, 2019, 12:44 »

Dear QW team,

When we build a two probe device... The device consists of electrode regions and electrode extension regions.

What is the purpose of this Electrode extension region ??
Is it to mitigate the effect of a practical contact ?

Please clarify.

2

General Questions and Answers / Different band structure of Graphene for 6 repetitions

« on: August 18, 2018, 08:21 »

Dear Experts,

I was calculating the band structure of graphene by repeating the hexagonal primitive cell for testing purposes.
I repeated the hexagonal primitive cell 2, 3 , 4, 5, 6, 7 and 8 times, respectively.
I have received typical graphene band structure with Dirac point at the 'K' point of brillouin zone for 2, 3, 4, 5, 7, 8 repetition cases.
However, the 6 times repetition case shows a different band structure with Dirac point at 'G' point. (The images are attached below).

Why the band structure is different only for 6 repetitions case ?
Let us say it is a result of folded bands and shrinked brilllouin zone due to the repetition of supercell. Then why we are getting the typical band structure (having dirac point at 'k' point) for 7 and 8 repetition cases

Please explain.
Awaiting your suggestions.

3

General Questions and Answers / pH value - molecular dynamics

« on: December 4, 2017, 15:20 »

Dear Experts,

I am eager to know if there is any way to adjust the pH value of solution inside the supercell, while performing molecular dynamics.

Thanks.

4

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

« on: March 24, 2017, 13:39 »

Dear QW experts,

I have test simulated graphene (2 atoms) for the band gap and DOS. I observed a small band gap from the density of states plot.

I have attached the python script and the obtained DOS plot for your reference.

Please point, where am I doing wrong?

5

General Questions and Answers / Calculating Quantum Capacitance

« on: March 7, 2017, 14:22 »

How to calculate Quantum capacitance of a graphene sheet using ATK?
Can someone help me with this, plzz.

Thank you very much for your time.

6

General Questions and Answers / problem displaying Bandstructure of Graphene: VNL as QE GUI

« on: January 4, 2017, 19:43 »

Dear QW Team,

I am using VNL as GUI for Quantum Espresso. First, I implemented the tutorial of Silicon, VNL worked fine for me.

When I went on to Graphene (two atoms) bandstructure calculation, the VNL shows empty band structure plot. However, when I plot the same using GNUPLOT it shows perfect  bandstructure and zero band gap.

Why VNL could not show band structure of Graphene, however it is working fine in displaying DOS plot of graphene. I really wasted a lot of time thinking that the problem is with my calculation.
Please clarify.

Thank you very much for your time.

7

General Questions and Answers / Mulliken charges, Wrong Prediction by ATK

« on: September 25, 2016, 19:37 »

Dear QW team,
Wishes of the Day.

I like to report a problem that I encountered several times. ATK could not predict the mulliken charges correctly. Most of the times it gives completely wrong mulliken results.
For example, the mulliken charges extracted using ATK-DFT for NO2 molecule are
N  -0.123e  (gained)
O  +0.062e (lost)
O  +0.062e (lost)

The same using Gaussian09 DFT package gives,
N  +0.479e (lost)
O  -0.241e   (gained)
O  -0.238e   (gained)

This type of complete contradictory results in case of mulliken population were witnessed several times by me.

Awaiting your reply.
Shan 

8

General Questions and Answers / Thermoelectric properties for bulk fcc

« on: March 30, 2016, 08:33 »

Hello QW team...

I am trying to calculate the thermoelectric properties of BN (bulk FCC) structure. I went though the tutorial  https://quantumwise.com/documents/tutorials/latest/Phonon/index.html/chap.thermoelectric.html

However, to calculate these properties using this tutorial, phonon transmission spectrum and transmission spectrum are needed. Which i couldn't find in the script generator when i added BN bulk crystal.

Please tell me how can i calculate the thermoelectric properties for this bulk material.


Thank you very much for ur time...  and Wish you a wonderful day.

9

General Questions and Answers / "Area of cross section" of Atomic chain

« on: March 8, 2016, 16:28 »

Dear sir,

I want to calculate the resistivity (rho = R*A/l)) of an atomic chain, for that I need to calculate area of cross section first. I am really confused, on how to calculate the area of cross section for the atomic chain. (structure is shown in the figure)

plz help sir. Thank you very much for your time.

10

General Questions and Answers / Strain calculation from stress tensor

« on: March 2, 2016, 17:49 »

Dear sir,

I have applied tensile stress on BN nanowire by increasing the size of supercell in the periodic direction (z-direction).
The computed stress tensor in z-direction before and after applying stress are 0.0377 and 0.152 eV/A^3.

1) How can I calculate the strain tensor in z-direction from the above computed stress tensor?
2) The percentage increase in stress in the above case is 303 %, Is the percentage increase in strain will also be 303 % ? (I read in some sources that strain is directly proportional to stress) 


Please clarify sir.  Thank you very much for your time.

11

General Questions and Answers / error opening DOS in VNL

« on: January 30, 2016, 20:40 »

I could not open DOS in my VNL 2015.1 version. The following error occurs when i try to open DOS plot.
I am able access all other plots without any error but not DOS...

Please suggest...
thank you in advance.

12

General Questions and Answers / Bandgap and DOS contradicting in InSb nanowire

« on: July 27, 2015, 19:28 »

Hello sir,

I got contradicting band structure and density of states, when I simulated InSb nanowire using GGA-rPBE, 1*1*40 k-pts, DZDP.
the band gap shown semi-conducting with a bandgap of 0.14 eV. whereas the DOS shows high quantity of states on fermi level.

Then what is the nature of the InSb nanowire I simulated Semi-conducting or metallic?

I exported the DOS to a text file to check the fermi level, it shows fermi level is at -4eV. but the band gap from DOS plot at -4eV is much higher than 0.14 eV.

I am a bit confused after watching these results... Can u please suggest me what is the nature of the nanowire (metal or semi-conducting or semi-metallic) and why is it like this?

I attached the band structure, DOS and exported file of DOS for your reference....

Thank you sir.

13

General Questions and Answers / Gate ineffective in tunnel junction device, ?

« on: July 10, 2015, 14:05 »

Dear QW team,

I have test created a tunnel junction device and tried to plot its IV (current-voltage) characteristics at different gate voltages. I used anthracene molecule as a tunnel island. Used extended Huckel model to plot the IV curves.

Though I varied the gate voltage, no changes were observed in the IV plot. I observed same IV plot at gate voltage of 0V and 2V.

can u please explain me.... what is wrong here... and why the gate become ineffective in supplying energy to the island. (dielectric constant of dielectric layer is taken as 10 epsilonzero).

 Here I am attaching the configuration, and curves.

14

General Questions and Answers / metal type in metallic region (spatial region) of atk

« on: July 6, 2015, 16:41 »

Hello QW team,

what is the default metal it takes when we insert the metallic region using 'spatial region' option in atk builder. There is an option to change the dielectric constant when we insert the dielectric layer, but there is no option available to tune the work function or conductivity of the metal.

Then what is the  default metal used here... and how to change the metal type.

thank you...

15

General Questions and Answers / difference among the boundary conditions (Nuemann, dirichlet, periodic, MP)

« on: July 6, 2015, 16:09 »

Hello all

I have a long standing confusion regarding the usage of "boundary conditions" in "Multigrid Poisson solver". Even though I have some idea about some of the available boundary conditions, I have much confusion regarding which one is efficient for what kind of system and what they meant for.

If I am correct, Neumann is to create zero transverse electric field at the boundaries and used in gated devices. Periodic condition is for unit cell analysis of infinitely periodic systems.

some one please explain... what the Nuemann, dirichlet, periodic, Multipole conditions are stand for, and for what kind of systems each of these are suitable...

plz...