[CPMD-list] Dipole dynamics vs standard autocorrelation spectrum?

[Juerg Hutter]

Hi
> Hello all,
>
> I perform my CPMD calculation without DIPOLE DYNAMICS keyword, so now i
> don't have DIPOLE file and that implicate - I can calculate only "power
> spectrum". This spectrum is not IR spectrum, but rather all the
> vibrational degrees of freedom of the system. I suppose there is no
> possibility to obtain this DIPOLE file now. I think only possibility
> to get this file is to reapet this calculation with DIPOLE DYNAMICS
> keyword or maybe there is easier way?

Yes, you have to repaet at least part of the calculation.
You probably don't want to calculate the dipole at every CP step,
so you could only recalculate every 5th to 10th step using a
clever script.
There is also the MOLECULAR DYNAMICS PT keyword that is redoing
a MD from a TRAJECTORY file. However, I don't know what
properties are available with this option and how to correctly
use the option.

>
> >Perform a similar analysis on the dypole dynamics (keyword DIPOLE
> >DYNAMICS in a MD simulation) that is recorded in the file DIPOLE. In this
> >case you have the IR bands. Again you do not get the intensities.
>
> And now I confused.
>
> 1. DIPOLE DYNAMICS - I get IR band - really? But why I don't get the
> intensities? Are maybe some free scripts available to compute spectrum
> when I have DIPOLE file?

You get the intensities, apart from some constants. Have a look into the
papers by Bernasconi et al. or Sprik et al.
For the derivation of the formula look into McQuarrie "Statistical
Mechanics".

>
> 2. Normall I compute standard autocorrelation spectrum whitout DIPOLE.
>    How bad is this power spectrum when I compared it with DIPOLE spectrum?
>    Are some papers which deal with this topic?
>    Are intensities in standard autocorrelation spectrum really very bad
>    or maybe  ?
>
>

Again see McQuarrie. Intensities from the velocity autocorrelation is not
related to the absorption intensity.

>
> WANG wrote also:
> >Calculate the power spectrum from a MD calculation. In this way you get
> >rid of harmonic approximation, but you need a very very very long run to
> >have a sufficient accuracy in numerical data.
>
>
>
> What means very very long run ---> 50000 steps or 70000 steps. Is this
> enough?
>
>

Long wrt to the important frequencies you want to study. The lower
the frequencies the longer you have to sample.
I would take about 20ps for frequencies above 1000 wavenumbers as
sufficient.


> 3. I computed power spectrum from Path-Integral dynamic.
>    I'm interesting in hydrogen bond. In this dynamics I get center
> position of hydrogen in hydrogen bridge --> standard autocorrelation
> spectrum show continuum of absorbtion 1700 cm-1 to 800 cm-1, in the
> standard Car-Parrinello Dynamics spectrum I didn't got this continuum.
> Is this contunuum maybe effect of PI dynamics(I did the same what Axel
> Kohlmayer script - I computed average of replicas - obtained centroid -
> so spectrum of centroid).
>

I can't answer this questions. But my guess is that the velocity
autocorrelation from a path integral MD calculation is not
(at least simply) related to the IR spectra.

regards

Juerg Hutter