[Pw_forum] input for d3.x

[Stefano Baroni]

Hi! To my understending, vibronic linewidths in any finite system (such as the CO2) cannot have anything to do with anharmonic effects. In an infinite system, the anharmonic contribution to a phonon linewidth is due to the (anharmonic) coupling between a state of interest and a *continuuum* of quasi-degenerate harmonic states. In the symple case of a Raman or IR line, the state of interest is a zone-center (IR or Raman) active mode, while the continuum is made by pairs of acoustic modes of opposite momenta and frequencies whose sum is equal to the frequency of the optic mode.  In a molecule, all the harmonic modes are discrete, so that no coupling among them can cause any broadening. SB

On Jul 29, 2010, at 2:12 PM, Martin Andersson wrote:

> Hi all,
> 
> I was trying to find information on how to use the d3.x program. I have
> successully calculated frequencies, ir and raman intensities for my test
> system, a CO2 molecule. I would like to try and calculate anharmonic
> effects on linewidths using d3.x but can't seem to get it to work. If
> anyone could point me in the right direction I would be grateful.
> 
> Thanks,
> Martin Andersson,
> NanoGeoScience,
> University of Copenhagen
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
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