[DFTB-Plus-User] Failed geometry optimisation for Au atom cluster

[Ben Hourahine]

Hello,

you still have not explained why you are using LDA+U. This is likely
to be physically wrong for these systems (closed d-shell).

Tiem Leong Yoon wrote:
> Dear colleagues of DFTB+,
> 
> We have posted this problem before and received some feedback. Since
> then, we make some progress and now we re-post the problem and request
> for some input...
> 
> We have constructed a Slater-Koster file for gold (Au) nanocluster and
> run on dftb+. The Slater- Koster information is the courtesy of Prof.
> Koskinen, author of the paper "Density-functional based tigh-binding
> study of small gold cluster", P. Koskinen et al 2006, New J. Phys. 8, 9.
> 
> First we try with 4 and then 5 atoms. The initial structures are
> obtained from molecular dynamics simulation. All the structures are
> optimized through optimizer like Conjugate Gradient, in the DFTB+. It is
> found that the geometry will be optimized in less than 100 cycles.
> 
> But when we try to do with cluster of 10 atoms, the geometry
> optimization failed as after 1000 cycles, the geometry still did not
> optimized. Of course, each cycle of geometry optimization is done with
> self-consistency, i believed.
> 
> The above optimization is done with "threshold value of force component"
> of 1e-4. However, if we raise threshold value to 5e-4, all the
> geometry(10 atoms of Au) optimization are achieved. 
> 
> We also try with 38 atoms of gold cluster. The geometry optimization is
> successful if we use threshold value of force component of 6e-4. For
> your info., for 38 atoms of Cu cluster, the geometry optimization is
> successful for  threshold value of force component of 1e-4. 1e-4 is the
> standard value used in DFTB+ .
> 
> We try with threshold value of force component of 1e-4, and try the
> below mentioned, and the geometry optimization failed after 1000 cycles:
> 
> 1) we implement LDA+U corrections by inserting function
> "OrbitalPotential" available in DFTB+. 
> 
> 2) we raise the fermi temperature from 0 to even 500. 
> 
> 3) No spinpolarization, i.e spinpolarization = {}.  "
> 
> So, our question is, is this problem DFTB+ -specific? 
> 
> thanks
> 
> regards
> 

-- 
     Dr. B. Hourahine, SUPA, Department of Physics,
   University of Strathclyde, John Anderson Building,
          107 Rottenrow, Glasgow G4 0NG, UK.
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