[CP2K:2897] IR spectr of X-D2O: discrepancy from band analysis of stretches

Fawzi Mohamed fa... at gmx.ch
Tue Nov 2 11:13:56 UTC 2010

On 1-nov-10, at 19:13, jadefox337 wrote:

> Hello,
> I am getting IR spectrum from system dipole DFT (BLYP) for cellobiose
> + 1
> D2O.  For D I used same basis sets as for H but with double the
> mass.   I
> also compute specific band stretches frequencies for D2O (from FFT of
> the
> bond length along the trajectory).  These should match quite nicely,
> but
> in fact there is quite a large discrepancy - about 100 cm-1.
> Specifically:
> (a)   DO-free (band analysis) ~ 2730 cm-1  versus DO-free (dipole
> spectrum) ~ 2640 cm-1
> (b)  DO - h-bonded (band analysis) ~2440 cm-1 versus DO-h-bonded
> (dipole
> spectrum) ~ 2340cm-1
> Does anyone have any explanation ?  Could there be some error / bug in
> dipole computation or in the way I used the basis set for D ?

I don't understand so well what you are comparing with what.
If you compare harmonic bands with the ones from the trajectory then  
the difference are anharmonic effects.
If you are comparing to experiment, well DFT has an error, and on the  
top of it normally its frequencies are shifted from the correct ones.

> Thanks much for your help,
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