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) > > > La morale est une logique de l'action comme la logique est une morale > de la pens?e - Jean Piaget > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum -- 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
