[DFTB-Plus-User] transmission (conductuvity) very low

[toufik essakhri]

Dear all,
I am running transport calculations for Ti2C MXene which is showing very strong metallic character from band structure calculations and also from literature.
However, the calculated transmission coefficients are very low (in the order of 10-20~10-16), and therefore low conductivity.
I found a DFT study on the same material, in which authors found transmission coefficients in the order of 2.

What can be the origin of this inconsistency?
Any hints are really appreciated

*) part of the gen file (transport is considered in the x-direction; Device (1-49) + Source (50-61) + drain (62-73))
# ==================================================================
73 S
C Ti
     1   1   1.0588690120E+01   0.0000000000E+00   8.6498450460E+00
     2   2   1.1471080411E+01   1.5283457040E+00   7.5081940050E+00
     3   2   1.2353471648E+01   0.0000000000E+00   9.7914976340E+00
     4   1   1.3235861940E+01   1.5283457040E+00   8.6498450460E+00
     5   2   1.4118253177E+01   0.0000000000E+00   7.5081940050E+00
.....
.....
    70   1   6.0884965111E+01   1.5283457040E+00   8.6498450460E+00
    71   2   6.1767356349E+01   0.0000000000E+00   7.5081940050E+00
    72   2   6.2649747587E+01   1.5283457040E+00   9.7914976340E+00
    73   1   6.3532145185E+01   0.0000000000E+00   8.6498450460E+00
        0.000000000         0.0000000000         0.0000000000
       78.532145185         0.0000000000         0.0000000000
        0.000000000         3.0566914082         0.0000000000
        0.000000000         0.0000000000        17.2996921539
# ==================================================================
*) input for contact calculation
# ==================================================================
Geometry = GenFormat {
  <<< "str_Ti2C_trans.gen"
}
Transport {
  Device {  AtomRange = 1 49  }
  Contact {
    Id = "Source"
    AtomRange = 50 61
    PLShiftTolerance = 0.0003
  }
  Contact {
    Id = "Drain"
    AtomRange = 62 73
    PLShiftTolerance = 0.0003
  }
  Task = ContactHamiltonian { ContactId = "Source"  }
}
Hamiltonian = DFTB {
  SCC = yes
  MaxSCCIterations = 5000
  EwaldParameter = 0.1
  SCCTolerance = 1e-5
  Mixer=Broyden{MixingParameter=0.001}
  SlaterKosterFiles = Type2FileNames {
    Prefix = "/home/executables/my-set/"
    Separator = "-"
    Suffix = ".skf"
  }
  MaxAngularMomentum {
   Ti = "d"
    C = "p"
  }
  Filling = Fermi { Temperature [Kelvin] = 500.0 }
  KPointsAndWeights = SupercellFolding {
    1   0  0
     0  9  0
     0  0  1
    0.0 0.0 0.0
  }
  Solver = DivideAndConquer { }
}
# ==================================================================
*) input for device calculation
# ==================================================================
Geometry = GenFormat {
  <<< "str_Ti2C_trans.gen"
}
Transport {
  Device { AtomRange = 1 49  }
  Contact {
    Id = "Source"
    AtomRange = 50 61
    PLShiftTolerance = 0.0003
    FermiLevel [eV] = EF_s #Fermi source (extracted from detailed_source.out)
    Potential [eV] = 0.0
  }
  Contact {
    Id = "Drain"
    AtomRange = 62 73
    PLShiftTolerance = 0.0003
    FermiLevel [eV] = EF_d #Fermi drain
    Potential [eV] = -0.0
  }
  Task = UploadContacts {}
}
Hamiltonian = DFTB {
  SCC = yes
  MaxSCCIterations = 5000
  SlaterKosterFiles = Type2FileNames {
    Prefix = "/home/executables/my-set/"
    Separator = "-"
    Suffix = ".skf"
  }
  MaxAngularMomentum {
   Ti = "d"
    C = "p"
  }
  Filling = Fermi { Temperature [Kelvin] = 500.0 }
  KPointsAndWeights = SupercellFolding {
     1  0   0
     0  1   0
     0  0   1
    0.0 0.0 0.0
  }
  Electrostatics = Poisson {
    MinimalGrid [Angstrom] = 0.4 0.4 0.4
    AtomDensityTolerance = 1e-5
    CutoffCheck = Yes   # Make sure you know what you are doing!
    BuildBulkPotential = Yes
    SavePotential = Yes
    PoissonAccuracy = 1e-5
  }
  Solver = GreensFunction {
    Verbosity = 1
    SaveSurfaceGFs = No
    Delta [eV] = 1e-4
    ContourPoints = 30 40
    RealAxisStep [eV] = 0.025
    LowestEnergy [eV] = -40.0
  }
}
Analysis{
  TunnelingAndDos {
    verbosity = 0
    EnergyRange [eV] = -20.0  20.0
    EnergyStep [eV] = 0.02
    Delta [eV] = 1e-4
    ComputeLDOS = Yes
    WriteTunn = Yes
    WriteLDOS = Yes
  }
}
# ==================================================================
Regards,
Toufik

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