[Pw_forum] Problem with Raman and Infrared Active Modes

[Stefano Baroni]

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.

Enjoy!
Stefano B

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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 pensée - Jean Piaget

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