[Pw_forum] Problem with Raman and Infrared Active Modes

[Andrea Dal Corso]

On Fri, 2011-10-07 at 16:33 +0200, Stefano Baroni wrote:
> Dear Hasan,
> 
> 
> > Actually, being  R (or IR) active of acoustic modes arouses
> > suspicion about calculation.
> 
> 
> I do not know how the IR/Raman character of lattice vibrations is
> assigned in QE (Andrea Dal Corso may want to comment on this). All I
> can say is that symmetry analysis alone can only say if a mode *may*
> be Raman. IR active or not, *NOT* if it actually is. (i.e. it can only
> say which modes are inactive, not which ones are active). The
> resulting intensity may be exceedingly small (or even rigorously
> vanishing) for dynamical reasons that have nothing to do with
> symmetry. For instance, zone-center acoustic modes are inactive
> because of translational symmetry, which can hardly be detected by
> point-symmetry arguments. This is the case even for finite systems.
> Take any polar molecule (H2O, CO, ...) as an example. A rigid
> translation of the molecule (the 3 independent such modes) has zero
> frequency and is IR and Raman inactive. I do not believe that such
> inactivity can be predicted by any rotational symmetry arguments ...
> 
> 
> In any event, I think that if the code is incorrect, it should be easy
> to demonstrate. Just take the table of characters of the symmetry
> group of the system you are interested in, and assign the IR/Raman
> activity of each mode by yourself. It should not be difficult to do,
> and it is very instructive to do once in one's life, if you have never
> done it. If it turns out that the activities reported by QE are
> incorrect (either because the symmetry group is incorrectly assigned,
> or because the activity for a given group are incorrectly predicted),
> please revert to us with a detailed report.
> 

This is actually what the code does. It puts a label close to the
irreducible representation. The label is taken from the tables of
characters for irreducible representation (if x,y, or z are basis
functions for the representation it puts IR, if  xy, or xz, ecc. are
basis functions it puts R, if it has basis of  both types it puts R+IR).

HTH

Andrea

> 
> Enjoy!
> Stefano B
> 
> ---
> Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -
> Trieste
> http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) /
> stefanobaroni (skype)
> 
> 
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> de la pensée - Jean Piaget
> 
> 
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-- 
Andrea Dal Corso                    Tel. 0039-040-3787428
SISSA, Via Bonomea 265              Fax. 0039-040-3787249
I-34136 Trieste (Italy)             e-mail: dalcorso at sissa.it