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

[Salomon Billeter]

Hi Przemyslaw,

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

> Maby unfortunately I said: centroid.

your statement was right, you referred to the 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?

Although you used the right variable, the centroid, imaginary-time path
integral MD does not directly produce real-time dynamics of the centroid
for the reasons I mentioned. In principle, you can use an analytic
continuation to derive time-dependent properties from the ensemble
generated (cf. canonical density operator \exp(-\beta\hat H)/Z vs.
propagator \exp(-i\hat H t)), but in practice, you'll want to use the
adiabatic ab initio centroid molecular dynamics method.

You can read about this method (keyword CENTROID DYNAMICS in the &PIMD
section) in Marx, Tuckerman and Martyna, Computer Physics Communications
118, 166 (1999). The manual gives details about specific requirements of
the method such as normal mode propagator (which you already use) etc.

Best regards,
Salomon