[Pw_forum] About the mean square displacement in CP output

[Antonio Tilocca]

Dear Bertrand:

I am not sure I understand how you are extracting the MSD(t) from the CP
trajectory: are you just plotting the final values shown in the standard
output file, or (as it would be recommended) calculating them by
post-processing the trajectory file? In any event, a flat, oscillating
MSD(t), after an initial increase, is exactly what you would
expect for a solid.

Antonio Tilocca
UCL



>From siyouber at yahoo.fr  Thu Apr 29 15:09:27 2010
>From: siyouber at yahoo.fr (Bertrand SITAMTZE)
>Date: Thu, 29 Apr 2010 13:09:27 +0000 (GMT)
>Subject: [Pw_forum] About the mean square displacement in CP output

>I am running CP dynamic with fixed temperature for a given solid system,
>using cp.x. The microcanonical calculation went very well with a
>conserved energy.Now the energy is still conserved. But my emphasis is on
> the mean
>square displacement(MSD) of atoms. They are calculated and plotted as a
>function of time. At the begining, the dependence seems to be linear at
>after, the MSD seem to oscillate around certain values which can be taken
>as a mean value after a sufficiently long time. My questions are: 1)
>According to the litterature, the dependence at the beginning should be
>quadratic and became linear after a long time. But for condensed system
>as
>a solid it is, the MSD should saturate after a very long time. What is
>happening for my case? 2) The MSD computed with cp.x are tree
>dimensionnal. Please, confirm it or not. 3) In which file should I see
>how
>those MSD are computed?