[Pw_forum] About the mean square displacement in CP output

[Sandro Scandolo]

Dear Bertrand:

1) The MSD is quadratic in time at the beginning if you start with 
finite velocities. If you start with zero velocities the MSD will be 
even slower (t**4 in principle). For a fluid, the asymptotic behavior is 
linear in t,  for a solid it converges to a finite value (which can be 
connected to the Debye-Waller factor of x-ray intensities) in a time 
scale comparable to the vibrational time scale, so not too slow.
2) The MSD computed in cp.x are 3-D.
3) MSD's are computed in subroutine "ions_displacement" which can be 
found inside "ions_base.f90" in directory "Modules".

Your simulation looks ok, except perhaps for the linear dependence in t 
at the beginning, but the quadratic (or other) initial laws are 
generally seen only in the first very few steps, 10-20 steps are more 
than enough to spoil the ideal behavior typically.

Sandro


>
>
> 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?
>
>
> Thanks very much for your help
>
>
> ***************************
> Bertrand SITAMTZE YOUMBI
> Laboratory of Material Sciences
> Department of Physics
> University of Yaounde I-Cameroon
> ****************************
>
>
>
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-- 
Sandro Scandolo, ICTP | http://www.ictp.it/~scandolo

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