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

[Przemyslaw Dopieralski]

Hi Salomon,

I didn't use CENTROID DYNAMICS.
Last time I sent you my input which i use.

Maby unfortunately I said: centroid.
But on the othe side this is a some kind of centroind because I obtain it
from (in my case) 3 replicas. So I interpet only this average (I add up
position and velocities vector in every step and then I devide it by 3) of
this 3 replicas. But if I understand you well, I cannot obtain so easily autocorrelation spectrum?
Now the question is - how to do it?


&PIMD
   TROTTER DIMENSION
     3
   NORMAL MODES
     1.d0
   FACMASS
     1.d0
   DEBROGLIE CENTROID
     500.d0
   PRINT LEVEL
     1
&END

best regards,
Przemek



On Mon, 4 Jul 2005, Salomon Billeter wrote:

> Hi Przemyslaw,
>
> > 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).
>
> Did you use ab initio centroid molecular dynamics (keyword CENTROID
> DYNAMICS in the &PIMD section; see Marx, Tuckerman and Martyna in Computer
> Physics Communications 118, 166 (1999))? Otherwise, real-time information
> is not readily available in the "dynamics" of the centroid.
>
> In its standard formulation (imaginary time), path integral molecular
> dynamics does not represent - despite its name - the real-time dynamics of
> the system but instead cleverly samples the configurational space of the
> quantum system using an isomorphism between an n-times discretized path
> integral partition function of the system considered and a "partition
> function" of an n-times replicated classical system with scaled and
> excluded interactions and - importantly - a part of the kinetic energy term
> appearing as harmonic springs in the potential energy term. Therefore,
> (again, in the usual imaginary-time formulation), you cannot easily get
> real-time information such as velocity autocorrelation functions etc.
>
> Best regards,
> Salomon
>
>