[Pw_forum] Phonon omega or nu? Factors of 2pi

Saurabh Bajaj sbajaj at caltech.edu
Fri Sep 21 16:02:25 CEST 2012

Yes, you are right. The numbers with units /cm are wavenumbers (nu-bar).
Frequency, nu (/s) = c (cm/s) * nu-bar (/cm)
Lamda (cm) = 1/nu-bar (/cm)
Energy = hbar (energy-s) * nu (/s)

Saurabh Bajaj
California Institute of Technology

On Fri, Sep 21, 2012 at 9:25 AM, Johannes Moeller <
j.moeller1 at physics.ox.ac.uk> wrote:

>  Hello all,
> A quick question about the definition of the phonon frequency omega. Is
> the omega output by Espresso an angular or a linear frequency (omega/2pi)?
> I.e. is this converted into an energy by multiplying by hbar or h?
> Comparing simple vibrational modes of molecules (eg. of CO2) in cm^-1 to
> experimental values gives good agreement with experimental data, which are
> usually referred to as a liner frequency nu or 1/lambda rather than an
> angular frequency omega. This suggests that the value that Espresso outputs
> is nu and not omega(2pi*nu) as the name implies?
> The values output by Espresso in THz and cm^-1 seem to be related simply
> by a factor of c (no factors of 2pi). I'm using QE 5.01 but I don't think
> this is version specific.
> Many thanks,
> Johannes
>   -------------------------------
> Johannes Moeller
> Department of Physics
> University of Oxford
> Parks Road
> Oxford OX1 3PU, UK
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