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

[Przemyslaw Dopieralski]

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?

I read what wrote WANG YIXUAN:


>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?

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  ?



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?


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'm waiting for some advice.
thanks



PhD student
Przmyslaw Dopieralski
University of Wroclaw
Poland