[Pw_forum] the procedure of calculating the Raman tensor using the finite electric field method

Serge Nakhmanson nakhmanson at anl.gov
Thu May 31 23:17:00 CEST 2007

Dear Stefano,

You are, of course, correct, but what you are saying is so
obvious that I simply omitted this moment in my comment.

With the "quench-from-the-melt" technique it usually takes forever
to converge to zero forces and stress for a decent-size model that
simultaneously has accurate RDF and bond/dihedral angle distribution
functions. Large residual forces and strain unequivocally point to
a problem with model's structural relaxation. However, if one does
not want to spend forever relaxing it properly, *my advice* is to
use a model made with the Wooten-Winer-Weaire algorithm as an
initial guess and dance from there.

Naturally, a really small model would be easier to relax but
at the same time it would be useless for the studies of disorder.

I'm sorry that my comment might have been vague.


Stefano Baroni wrote:
> Just a brief comment on a recent note by
> On May 31, 2007, at 7:56 PM, Serge Nakhmanson wrote:
>> Just a small comment. Irregardless of schoolmate connections,
>> a "quench-from-the-melt" approach is not a good way to create
>> models of tetrahedrally bonded CRNs (continuous random
>> networks), which, among other things, might explain why your
>> stress and forces are so large. 
> I am afraid this is not correct. Whether or not a model for disordered 
> system obtained from a simulated quench is correct (this will depend on 
> the size of the model and the speed of the quench), the atomic forces 
> and overall stress should vanish if the quench has been performed in a 
> numerically accurate way. 

  Serge M. Nakhmanson               phone: (630) 252-5205
  Assistant Scientist                 fax: (630) 252-4798
  MSD-212, Rm. C-224
  Argonne National Laboratory
  9700 S. Cass Ave.
  Argonne, IL 60439

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