[Pw_forum] Raman and IR spectra

[Ari P Seitsonen]

Dear Gul and Eduardo et co,

   Just from the physical point of view, the problem is that if the photon 
energy in the experiment is larger than the band (or HOMO-LUMO, in a 
molecule) gap the electronic excitations are also possible, and not only 
the phonon scattering. Therefore the non-resonant formalism implemented in 
QE is no longer valid. The resonant Raman is then much more difficult a 
problem to handle; if you want to see some nice attempts to treat it 
appropriately, see for example the recent publications of Francesco Mauri 
and co-workers on the topic (in graphene at least).

   Whether the Raman signal _can_ be calculated using the non-resonant 
formalism when there is no band gap or the HOMO is half-occupied is 
another thing, which I do not want to attempt to answer here - yes, it 
will give it a number, but I am not qualified to say whether it has 
anything to do with the physical reality. :)

     Greetings from Rabat,

        apsi

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   Ari Paavo Seitsonen / Ari.P.Seitsonen at iki.fi / http://www.iki.fi/~apsi/
   Ecole Normale Supérieure (ENS), Département de Chimie, Paris
   Mobile (F) : +33 789 37 24 25    (CH) : +41 79 71 90 935


On Thu, 29 Oct 2015, Eduardo Menendez wrote:

> I do not see an obvious impossibility to calculate the IR and Raman spectrum of a cluster made of metal elements. A cluster is not a metal, just treat it like a molecule.
> A metal is a periodic quasi-infinite crystal with zero gap between occupied and empty levels.
> 
> If the cluster  has an open shell configuration , i.e., the HOMO is partiallly occupied, then it may not be possible to calculate IR and Raman with QE. However, if this
> were the case, the cluster would probably undergone a distortion (Jahn Teller effect) and will open a gap.
> 
> 
> Eduardo Menendez Proupin
> Departamento de Fisica, Facultad de Ciencias, Universidad de Chile
> URL: http://www.gnm.cl/emenendez
> 
> “No cometerás actos impuros ni publicarás en revistas open-acces”
> 
> 
> 
> On 10/27/15, Gul Rahman <gulrahman at qau.edu.pk> wrote:
> > Dear All,
> > Is it possible to calculate the Raman and IR spectra of metallic clusters
> > with QE. I heard it is possible to calculate it for semiconductors, but not
> > for metallic.
> > Why QE is not able to calculate IR and Raman spectra of a metallic system.
> > Thanks,
> > Gul
> >
> >
> > --
> >
> >
> > Dr. Gul Rahman
> > Assistant Professor,
> > Department of Physics,
> > Quaid-i-Azam University,
> > Islamabad, Pakistan
> > http://www.qau.edu.pk/profile.php?id=818020
> >
> 
> -
>