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

Johannes Moeller j.moeller1 at physics.ox.ac.uk
Fri Sep 21 15:25:37 CEST 2012

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 Moeller
Department of Physics
University of Oxford
Parks Road
Oxford OX1 3PU, UK

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