[CPMD-list] MD at fixed tempeature

[Ari.P.Seitsonen at iki.fi]

Dear Dr Jkhurdakar,

> Thanks for suggestions.  How much fluctuation from
> the desired Temperature is generally accepted? For
> example, for simulation at 300K for a system
> containing
> about 8 atoms, are the fluctuation of +-70K around
> 300K acceptable?
>   If I've correctly understood your mail, I use the
> following input for CPMD section to get the desired
> initial TEMP.
> 
>  & CPMD
>   MOLECULAR DYNAMICS
>    QUENCH BO
>   TEMPERATURE
>    300
>    MAXSTEP
>     10
>   &
>   Then I rerun the simulation with the following
> section.
> 
>   & CPMD
>    MOLECULAR DYNAMICS 
>    RESTART WAVEFUNCTION COORDINATES VELOCITIES GEOFILE
>     QUENCH BO
>     MAXSTEP
>      1000
>    &END
>  And perform the averages after discarding first few
> picoseconds for equilibration. Is this the correct way to 
> siumlate an isolated system at 300K with CPMD.

First of all, in general you should not quench the electrons to the
Born-Oppenheimer surface in Car-Parrinello dynamics, in the latter
input when you do the restart.

  Secondly, the average fluctuation in microcanonical simulation is
about dT = sqrt[2/(3*N)] * T_average, where N is the number of atoms.
So in your case, provided that you start from an equilibrated geometry
the final temperature would be around 300 K (please notice that for
this you should have the initial temperature 600 K!), and the
fluctuation ca. 87 K.

    Greetings from Rome,

       apsi

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