[DFTB-Plus-User] SCC and Spin Relaxation

[José Manuel Vásquez Pérez]

Dear users of DFTB+

I just noticed that the SCC does not always converge to the same energy
when optimizing iron clusters with a high number of unpaired electrons,
depending on the initial charges or the initial number of unpaired
electrons (if spin relaxation is allowed). I need to know if it is a normal
behavior or if it is a bug in the code. I also noticed that spin relaxation
only works when the initial number of unpaired electrons is slightly
different than zero (for example for Fe6 it works with 0.1 initial unpaired
electrons but not with 0, and for Fe11 it does not work even with 0.5
initial unpaired electrons) despite the fact that the most stable
multiplicity is larger than zero. I would like to understand why it does
not optimize the number of unpaired when their initial value is too close
to zero. Thanks in advance for your help.

This is an example of my input:

******************************************************************************************
Geometry = GenFormat {
6 C
Fe
     1   1  -1.5133154160E+00  -3.1163699090E+00   4.8395525190E+00
     2   1  -2.2869331200E+00  -1.1577302380E+00   7.7144216990E+00
     3   1  -5.9171879890E-01  -2.5667263510E+00   6.9211489160E+00
     4   1  -1.0719373520E+00  -7.3916752260E-01   5.5466727030E+00
     5   1  -3.2012161060E+00  -1.7091401130E+00   5.6341844250E+00
     6   1  -2.7174110570E+00  -3.5378755450E+00   7.0109896540E+00
}

Driver = ConjugateGradient {
  MovedAtoms = 1:-1
  ConvergentForcesOnly = No
  AppendGeometries = Yes
  MaxSteps = 300
}

Hamiltonian = DFTB {
  SCC = Yes
  MaxSCCIterations = 50
  Eigensolver = RelativelyRobust {}
  SlaterKosterFiles = Type2FileNames {
    Prefix = "./"
    Separator = "-"
    Suffix = ".skf"
    LowerCaseTypeName = No
  }
  MaxAngularMomentum {
    Fe = "d"
  }
  SpinPolarisation = Colinear {
    UnpairedElectrons = 0
    RelaxTotalSpin = Yes
  }
  SpinConstants = {
    Fe = {
      #  ss      sp      sd     ps       pp      pd     ds      dp      dd
      -0.016  -0.012  -0.003  -0.012  -0.029  -0.001  -0.003  -0.001  -0.015
    }
    ShellResolvedSpin = Yes
  }
  Mixer = Broyden {}
  Filling = Fermi {
    Temperature [K] = 1.0
  }
}

ParserOptions {
  ParserVersion = 5
}
******************************************************************************************

José Manuel
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