Severe challenges with phonon transport

Hi there,

I’m trying to calculate the phonon transport for through molecules between two Au-electrodes. Yet, I’ve struggled for a long time to get transmission that looks correct.
As a sanity check, I’ve calculated the transmission for a bare 4x4 slab with two small pyramids:


Only the two small pyramids are in the central region. The separation between the two tips is ~23Å.

I don’t expect much, if any, transmission for this system yet I see significant transmission at ~0eV.
image

The force constants of my electrodes have been calculated with these options:

TimeReversalSymmetry false

%block kgrid.MonkhorstPack
 2 0 0  0.
 0 2 0  0.
 0 0 2  0.
%endblock

# This is the remaining default SIESTA options
PAO.BasisSize     DZP

XC.functional     GGA
XC.authors        PBE
MeshCutoff        300.000000 Ry

SCF.DM.Tolerance       0.0001
# Mixing parameters:
SCF.Mixer.Weight       0.10
SCF.Mixer.History      10
DM.UseSaveDM           true

MD.NumCGSteps 0

SaveHS                          T
Write.Forces True

I utilize the bloch expansion in the transmission calculation so the smallest unit of the electrode is:

   18
sisl-version=1 cell= 2.88499567 0.00000000 0.00000000 0.00000000 4.99695908 0.00000000 0.00000000 0.00000000 21.20030188
Au  0.00000000  0.00000000  0.00000000
Au  1.44249783  2.49847954  0.00000000
Au  1.44249783  0.83282651  2.35558910
Au  0.00000000  3.33130605  2.35558910
Au  0.00000000  1.66565303  4.71117820
Au  1.44249783  4.16413256  4.71117820
Au  0.00000000  0.00000000  7.06676729
Au  1.44249783  2.49847954  7.06676729
Au  1.44249783  0.83282651  9.42235639
Au  0.00000000  3.33130605  9.42235639
Au  0.00000000  1.66565303  11.77794549
Au  1.44249783  4.16413256  11.77794549
Au  0.00000000  0.00000000  14.13353459
Au  1.44249783  2.49847954  14.13353459
Au  1.44249783  0.83282651  16.48912369
Au  0.00000000  3.33130605  16.48912369
Au  0.00000000  1.66565303  18.84471279
Au  1.44249783  4.16413256  18.84471279

The electrode is calculated with a supercell expansion of 5x3x2.

I’ve read this from the mailing list talking about the need for large electrodes, but I imagined that the ones I’ve used would be large enough?

I’ve also tried with a bigger surface (6x6, 8x8, 10x10), but that doesn’t seem to help.

Do I just need to bite the bullet of calculating an even deep electrode? And if so, why? I would think that 23Å of vacuum would not transmit any phonons?

I sincerely hope someone can assist me as I’m at my wits end :frowning: