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

Serge Nakhmanson nakhmanson at anl.gov
Thu May 31 19:56:18 CEST 2007

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. There are better methods/models
available for Silicon and Carbon (I think, Normand Mousseau
has good models of various sizes):

*] F. Wooten, K. Winer, D. Weaire, PRL 54, 1392 (1985),
**] G.T. Barkema and N. Mousseau, PRB 62, 4985 (2000).

I would also suggest checking whether or not your model
is actually metallic. All the under/over-coordinated
defect states that you have in "quench-from-the-melt"
models tend to fill out their DFT band gaps. See,
e.g., Fig. 16 in Voyles et al, JAP 90, 4437 (2001)
and compare, e.g., to Fig. 4 in Nakhmanson et al, PRB 63,
235207 (2001). In a (relatively) small model that you
may be studying such states could have significant
overlap despite being localized.

If your model is indeed metallic, then you are in a
"crap in -- crap out" mode and whatever you might
compute for it cannot be fully trusted.

Of course, Car and Parrinello did a pioneering work in
1987, but the field of CRN modeling has evolved a bit
since then :))

Hope this helps (all of the above is IMHO),


li niu wrote:
> 1.I consider a model structure of tetrahedral amorphous carbon obtained 
> by my schoolmate using a Car-Parrinello molecular dynamics approach.   

  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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