Messages

1

General Questions and Answers / Re: The interaction energy of bulk electron-phonon coupling

« on: August 2, 2017, 15:31 »

Hi,
1) The value of 300 meV mentioned in PRB 93, 035414 (2016) is not a calculated interaction energy. Rather it is a shift of the Fermi energy that we specify in order to simulate doped graphene.
2) The Fermi velocity can be easily calculated with calculateVelocity method (http://docs.quantumwise.com/manuals/Types/calculateVelocity/calculateVelocity.html). The phonon velocities you need to calculate 'by hand' using the formula v = 1/hbar * d E(q) / dq, where E(q) is the energy of a particular phonon branch, and q is the norm of the phonon wave vector in Cartesian coordinates.

2

General Questions and Answers / Re: Tutorial of Inelastic electron tunneling spectrum

« on: August 19, 2016, 09:59 »

Hi,

You are right that the HamiltonianDerivatives and DynamicalMatrix calculations could share the same scf results. This is something we have planned to implement, but we have not done it yet.

Regarding the k-point sampling for HamiltonianDerivatives and DynamicalMatrix: This does not have to be the same as for the device calculation, step (1). In fact, if the system is repeated in any direction (either set manually or done automatically) you should always reduce the number of k-point in that direction correspondingly. Otherwise the calculations will become too heavy. So you are right, that you should setup a separate calculator for the HamiltonianDerivatives and DynamicalMatrix calculations.

Note that you can get the automatically detected repetition with the function
(nA,nB,nC) = checkNumberOfRepetitions(configuration)
before you run the HamiltonianDerivatives and DynamicalMatrix calculations.

Troels

3

General Questions and Answers / Re: COUPLING PARAMETER

« on: June 22, 2016, 09:16 »

The self-energy for a given electrode (Left or Right) is computed as Ʃ_L(E)=(H_CL - ES_CL) g(E) (H_LC - ES_LC), where g(E) is the surface Green's function of the semi-infinite (Left) electrode, and H_LC is the part of the Hamiltonian that couples the left electrode (L) with the central region (C), and likewise for the overlap matrix, S_LC.

If you are interested in the self-energy (matrix) you can get it like this:

Code

sigma_L = calculateSelfEnergy(device_configuration,  energy=0.0*eV,  contribution=Left)

Notice, that you can also get access to the Hamiltonian, Green's functions and other quantities as described in this tutorial : http://quantumwise.com/documents/tutorials/latest/LowLevelEntities/index.html/chap.intro.html#sect.intro.hs

The self-energy is a complex matrix. The real part gives rise to a shift of the molecular energy levels, while the imaginary part gives the broadening (finite life time) of the molecular energy levels.

Sometimes the coupling parameter (matrix) is referred to as Ґ_L(E) = i(Ʃ^t_L(E) - Ʃ_L(E)), which is responsible for the energy broadening.

Since Ґ is a matrix, that depends on energy, the actual broadening of a particular molecular orbital should be obtained from the width of the spectral function A(E) = G^r(E)( Ґ_L(E) +  Ґ_R(E)) G^a(E), where G^r/a(E) is the retarded/advanced Green's function, which can also be obtained (see link above).

4

General Questions and Answers / Re: possibility to decompose the phonon transmission into different modes?

« on: May 3, 2016, 14:15 »

Hi,

It is currently not possible to decompose the phonon transmission directly into different polarizations. One way forward might be to calculate the vibrational modes in either the electrode or the device configuration. This gives you (visual) information about the polarizations. You can then compare the energies of the vibrational modes with the energies used in the transmission function.

Troels

5

Scripts, Tutorials and Applications / Re: effective mass direction

« on: December 7, 2015, 09:44 »

Hi,

That would be in Cartesian coordinates.

Troels

6

Scripts, Tutorials and Applications / Re: effective mass direction

« on: December 3, 2015, 14:56 »

Hi,

In principle one should be able to obtain the effective mass in direction [hkl] from the effective mass tensor (lets call it M) as [hkl]*M*[hkl]^T.
I am not really clear about what 'output vector' you are referring to  - can you give a code example.

Note, that we have disabled the effective mass tensor functionality in ATK2015. The reason is that we had trouble in getting correct results around degenerate bands. This is, however not a problem if you specify either the 'direction_cartesian' or 'direction_fractional'.

Troels

7

General Questions and Answers / Re: Use ATK DFT to calculate the hydrogen atom energy levels

« on: October 8, 2014, 11:28 »

The value of -13.6 eV, which is the Hydrogen atom ground state energy cannot be accurately obtained from the molecular energy spectrum, which reports the eigenvalues of the Kohn-Sham hamiltonian. The total energy in DFT includes the Kohn-Sham eigenvalues, but there are additional contributions. Therefore, what you really need to calculate is the total energy (TotalEnergy analysis object) which will give you a value close to -13.6 eV.

8

General Questions and Answers / Re: Born effective charges

« on: June 10, 2014, 16:30 »

Hi,

The acoustic sum rule should ideally be fulfilled. It should be OK to use the experimental structure. I would suggest that you try to decrease the displacement of the atoms (delta_z in the tutorial http://quantumwise.com/documents/tutorials/latest/Polarization/index.html/chap.born.html), if you didn't already try that.

Troels