Messages

1

General Questions and Answers / Re: Why the conductance is independent on the length of nanowire?

« on: November 29, 2012, 16:40 »

Thanks!

The conductance in your calculation takes into account only the ballistic conduction.  What you expect is true for the  resistance of the Ohmic contact.

2

General Questions and Answers / Why the conductance is independent on the length of nanowire?

« on: November 28, 2012, 23:52 »

Hi everyone:
I calculated the conductance of Li-wires with different lengths and found that the values of conductance are almost the same. But as I know, the electrical conductivity is a material property and indepedent on the length and cross section of the material. However, the conductance, which equals conductivity*(cross section)/length, should be inversely proportional to the length. Could someone tell me why my calculated conductance is independent on the length?
Thanks very much! 

3

General Questions and Answers / Re: Is the gate added correctly?

« on: September 6, 2012, 16:05 »

OK. Thank you all the same!

It's a bit hard to tell, the system is after all rather artificial, but I guess it means you can't really expect to have a H+ ion in this place, in this system.

4

General Questions and Answers / Re: Is the gate added correctly?

« on: September 4, 2012, 16:52 »

Hi, I have tried other site shifts, but the results are still very strange. When no site shift is added, the mulliken charge for H is 0.903. When the site shift is +10, the value is 1.463; when -10, it is 1.751. +8: 1.299; -8: 1.257. +5:1.095;-5:0.951. +3:0.994;-3:0.903. The charge of H atom is always increased. Could you please give me other suggestions? Thank you so much!

It looks correct. But I think a shift of 15 eV is also not suitable, it's probably way too much, which may explain the strange results.

5

General Questions and Answers / Re: Is the gate added correctly?

« on: August 29, 2012, 16:56 »

Hi, thank you very much! Yes, I think the result should be like what you say, but what I get is weird. I have read the document you mentioned to add the site shift, but since the case is a little different, I'm not sure whether it is right in my case. Could you please help me check if the method I use to add the site shift is correct? 
Thank you so much!

You cannot instruct the H atom to be in a particular charge state, since the Mulliken population of each atom will be determined by the self-consistent state of the whole system. Thus the relevant quantity to compare to is not the isolated H atom, but the self-consistent state of the H atom in the given atomic environment. I am sure (or at least I very much hope) that if you compare the 3 calculations with +15 eV, 0 eV, and -15 eV, you will see a net additional positive/negative charge on the H atom in the two cases with a finite shift, compared to no shift.

6

General Questions and Answers / Re: Is the gate added correctly?

« on: August 28, 2012, 23:38 »

Hi, I have tried your suggestion, but another thing puzzled me. When I added an onsite shift of +15*eV on H atom, the  Mulliken Population Report for H is 1.745; when onsite shift is -15*eV, the value is 2.327. It seems that the charge of H should be larger than ~1.00 for one case and smaller than ~1.00 for another. Could you please give me some suggestions for this problem? Thank you so much!

That's probably a bit too violent I would suggest adding an onsite shift on the H atom instead (see http://www.quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/ref.atomicshift.html).

7

General Questions and Answers / Re: How to calculate the charge transfer?

« on: August 28, 2012, 16:51 »

Thank you very much! I will have a try.

Hmm, an alternative to the Mulliken population could be using the ElectronDensity.
A rough example I have made:

Code

ed = nlread('analysis.nc', ElectronDensity)[0]
cut = 20
s = ed[:, :, :].sum()
print ed[:,:, :cut].sum()/s
print ed[:,:, cut:].sum()/s

Where 20 is some grid number in the z-direction.

Also take a look at the manual entry for ElectronDensity

8

General Questions and Answers / Is the gate added correctly?

« on: August 28, 2012, 16:27 »

Hi everyone,
I'm now considering studying the conductance of a device with H+ involved. The geometry is included in the file Pt.py. In order to simulate the H+, I add a gate just on the bottom of the atom H. By adjusting the gate, the atom H can be turned into H+. But I'm not sure whether it is reasonable using this method. In addition, what boundary condition should I use in this case?
Any suggestion would be appreciated. Thank you very much!

9

General Questions and Answers / How to calculate the charge transfer?

« on: August 6, 2012, 03:00 »

Hi everyone:
In this topic, http://quantumwise.com/forum/index.php?topic=369.0, the author calculated the charge transfer in the system. I'm also studying the effect of the gate voltage on the charge of the central region in a two-probe system. I know that the mulliken population can give the charge of an given atom. Is there another way to calculate the charge transfer?
Any advice would be appreciated!

10

General Questions and Answers / Re: Puzzled about the transmission change when bias voltage is added?

« on: July 30, 2012, 15:48 »

Thanks for your advice!

11

General Questions and Answers / Puzzled about the transmission change when bias voltage is added?

« on: July 29, 2012, 17:33 »

Hi everyone,
I am puzzled about one issue in the two-probe system. When I add voltages of -0.5V and +0.5V to the left and right electrodes, respectively, the Fermi level of the left electrode is shifted up and  the right is shifted down. When calculating the transmission, I think it is more easier for the electrons to transfer from left to right, which should lead to a larger transmission at the Fermi level. But in my calculation, the value is decreased. On the other hand, when I add a voltage of +0.5V and -0.5V to the left and right electrodes, respectively, the value is increased. I'm puzzled by the results. Could anyone help me? Thanks!

12

General Questions and Answers / Re: Not physically reasonable in mulliken population

« on: July 27, 2012, 19:28 »

Hi nori:
I have tried the method you suggested. It just the same as GGA.PW91.
Thanks!

13

General Questions and Answers / Re: Not physically reasonable in mulliken population

« on: July 22, 2012, 17:24 »

Hi Nordland,
Thank you for your suggestion. Since that's always the way to treat a molecule in other software, I'm used to use that method. 
Yes, I find in ATK the way to treat a molecule. That's great! 

14

General Questions and Answers / Re: Not physically reasonable in mulliken population

« on: July 22, 2012, 03:28 »

Hi Nordland,
Thanks for your patient reply.
   (1) I put the molecule HCl in a large enough box so that there are no interactions among the neighboring molecules and treat it as a periodic bulk. I think this can simulate the situation of a molecule HCl.
   (2) Yes, you are right. In H-Cl the electron is not transfered to the Cl atom completly. In my original post, I got the result that H get partial electron from Cl. I think this is not reasonable. I have read the reference you recommended, the result therein is -0.17 for Cl and +0.17 for H. I finally got a similar result by changing the basis set in the input file.
Thanks again for your suggestions.

15

General Questions and Answers / Not physically reasonable in mulliken population

« on: July 21, 2012, 18:19 »

Hi everyone,
I want to use the ATK to calculate the mulliken population, so I did a simple test on a HCl molecule. The result is:
 Mulliken Population Report                                                   |
|                                                                              |
| ---------------------------------------------------------------------------- |
|                        |                                                     |
| Element   Total  Shell | Orbitals                                            |
|                        |                                                     |
|                        |      s                                              |
|   0   H   1.044  0.129 |  0.129                                              |
|                        |      s                                              |
|                  0.773 |  0.773                                              |
|                        |      y      z      x                                |
|                  0.142 |  0.058  0.042  0.042                                |
|                        | --------------------------------------------------- |
|                        |      s                                              |
|   1  Cl   6.956  0.090 |  0.090                                              |
|                        |      y      z      x                                |
|                  0.630 |  0.153  0.239  0.239                                |
|                        |      s                                              |
|                  1.674 |  1.674                                              |
|                        |      y      z      x                                |
|                  4.525 |  1.091  1.717  1.717                                |
|                        |     xy     zy  zz-rr     zx  xx-yy                  |
|                  0.037 |  0.002  0.002  0.008  0.000  0.025                  |
+------------------------------------------------------------------------------+
But as is know, for the HCl, the result should be H+ and Cl- and the electrons for H and Cl should be 0.0 and 8.0, respectively.
My input file is:
# -------------------------------------------------------------
# Bulk configuration
# -------------------------------------------------------------

# Set up lattice
vector_a = [10.0, 0.0, 0.0]*Angstrom
vector_b = [0.0, 10.0, 0.0]*Angstrom
vector_c = [0.0, 0.0, 9.999945]*Angstrom
lattice = UnitCell(vector_a, vector_b, vector_c)

# Define elements
elements = [Hydrogen, Chlorine]

# Define coordinates
cartesian_coordinates = [[ 5.       ,  4.36     ,  4.9999725],
                         [ 5.       ,  5.64     ,  4.9999725]]*Angstrom

# Set up configuration
bulk_configuration = BulkConfiguration(
    bravais_lattice=lattice,
    elements=elements,
    cartesian_coordinates=cartesian_coordinates
    )

# -------------------------------------------------------------
# Calculator
# -------------------------------------------------------------
#----------------------------------------
# Basis Set
#----------------------------------------
basis_set = GGABasis.DoubleZetaPolarized

#----------------------------------------
# Exchange-Correlation
#----------------------------------------
exchange_correlation = GGA.PW91

numerical_accuracy_parameters = NumericalAccuracyParameters(
    k_point_sampling=(10, 10, 10),
    )

calculator = LCAOCalculator(
    basis_set=basis_set,
    exchange_correlation=exchange_correlation,
    numerical_accuracy_parameters=numerical_accuracy_parameters,
    )

bulk_configuration.setCalculator(calculator)
nlprint(bulk_configuration)
bulk_configuration.update()
nlsave('HCl.nc', bulk_configuration)

# -------------------------------------------------------------
# Mulliken population
# -------------------------------------------------------------
mulliken_population = MullikenPopulation(bulk_configuration)
nlsave('HCl.nc', mulliken_population)
nlprint(mulliken_population)

Could anyone give me any suggestions? Look forward to your reply! Thanks!