[DFTB-Plus-User] SCC and Spin Relaxation

Ben Hourahine benjamin.hourahine at strath.ac.uk
Thu Dec 7 09:47:20 CET 2017


Hello José Manuel,

unfortunately this is in part behaviour inherited from DFT to DFTB. For
spin free calculations there is a single, global, energy minima (in DFT
the energy is upwardly convex). But if you have spin resolved density
functionals this no longer holds (proof by Eschrig and Pickett back
around 2001) and there can be multiple global and local minima.

The reason for requiring a non zero spin is to break a symmetry in
paramagnetic systems. For example with an Fe2 dimer, there are two
possible ground state spin configurations:

^
|
Fe

^
|
Fe


and

Fe
|
v

Fe
|
v

so starting from the symmetric configuration with 0 spin on each atom,
the initial configuration is at a saddle point and will not
spontaneously magnetise (derivative of energy with spin at the
diamagnetic point being 0).

For Fe11 it sounds as though there is a weak local minima for at the 0
spin case.

Regards

Ben

On 07/12/17 03:46, José Manuel Vásquez Pérez wrote:
> 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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-- 
      Dr. B. Hourahine, SUPA, Department of Physics,
    University of Strathclyde, John Anderson Building,
            107 Rottenrow, Glasgow G4 0NG, UK.
    +44 141 548 2325, benjamin.hourahine at strath.ac.uk

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