[DFTB-Plus-User] Query on the use of DFTB+ for periodic transport calculations

[Karen Schafer]

Dear mailing list members,

I am relatively new to using DFTB+ and I can't fix a problem with 
self-consistent, periodic transport calculations. I am investigating a 
system with two gold (Au(111)) surfaces, bridged by an alkanedithiol 
ligand. The system is supposed to be periodic in x and y direction and I 
added a vacuum gap in z direction (transport direction).

When calculating the contacts, I receive a fermi energy of -2.6 eV which 
is higher than I would expect and also higher than what I get from 
non-periodic conditions (-4.2 eV), which results in a shift of the 
transmission compared to non-periodic and non SCC calculations.

I have included the gen structure file and the dftb_in file for the 
contact calculation. I have tried to fix the fermi energy by increasing 
the kpoints but that did not help. Is there something I am doing wrong?

Thank you in advance for your time and effort,

Karen

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Geometry = {
  GenFormat {
  <<< 'input.gen'
  }
}

Transport {
    Device {
      AtomRange = 1 173
    }
    Contact {
      Id = "source"
      AtomRange = 174 317
    }
    Contact {
      Id = "drain"
      AtomRange = 318 461
    }
  Task = ContactHamiltonian{
    ContactId = drain
  }
}


Driver = {}

Hamiltonian = DFTB {
  SCC = Yes 
  SCCTolerance = 1e-6
  MaxSCCIterations = 200
  MaxAngularMomentum = {
    S = "d"
    C = "p"
    H = "s"
    Au = "d"
  }

  Filling = Fermi {
    Temperature [Kelvin] = 0 
  }
  SlaterKosterFiles = Type2FileNames {
   Prefix = "/home/fcvx064/bin/DFTBplus/slater_koster/auorg-1-1/"
   Separator = "-"
   Suffix = ".skf"
  }
  KpointsAndWeights = SupercellFolding{
    4    0    0
    0    4    0
    0    0    1
    0.5  0.5  0
  }	
}
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