[CPMD-list] MD at fixed tempeature

[R Jkhurdakar]

Dear Apsi,

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.

 Thanks once again,

  Regards,

  RJ
 


--- Ari.P.Seitsonen at iki.fi wrote:
> 
> Dear Dr Jkhurdakar,
> 
> >   I'm interested in a molecular dynamics
> simulation
> > of a molecule at 300K. I'm somewhat consfused with
> 
> > the choice of appropriate KEWYWORD to achieve
> this.
> >  I want to calculate averages at 300K. So I'm
> using
> > the 
> > following input for the CPMD section. 
> >  &CPMD
> >     MOLECULAR DYNAMICS CP
> >     QUENCH BO
> >     EMASS
> >      800
> >     TIMESTEP
> >      25
> >     TEMPERATURE
> >      300
> >     TRAJECTORY SAMPLE
> >      10
> >     MOVIE
> >      5
> >  &END
> > 
> >    My question is if the about input is correct or
> > should I use  the keyword TEMPCONTROL IONS
> followed
> > by 300 50 on the next line?
> >  Any help or comments would be appreciated.
> 
> First of all, if your cluster is not very large, the
> temperature will
> fluctuate quite much during the simulation; you can
> decrease the
> fluctuations somewhat by using a Nose thermostat,
> but this might lead
> to somewhat unphysical trajectories as well.
> 
>   Secondly, if you specify only "TEMPERATURE" (same
> as "TEMPERATURE
> IONS") this only specifies the initial temperature,
> if the initial
> structure was relaxed to the global minimum you'll
> end up at around
> half of the initial temperature specified due to
> equilibration between
> kinetic and potential energy. So you can use
> "TEMPCONTROL IONS", but
> please notice that the temperature will fluctuate
> around the average,
> and you shouldn't take physical averages of
> trajectories with the
> rescaling switched on, i.e. the "TEMPCONTROL IONS"
> should only be used
> in the beginning to equilibrate the system.
> 
>   Hopefully this helps somewhat.
> 
>     Greetings,
> 
>        apsi
> 
> -- 
>
-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-
>  Ari Paavo Seitsonen / Ari.P.Seitsonen at iki.fi /
> http://www.iki.fi/~apsi/
>  Tel +41 1 635 44 97 / Fax +41 1 635 68 38 / GSM +41
> 79 719 09 35      
>  Anschrift: Physikalisch Chemisches Institut (PCI),
> Universität Zürich (UniZh)
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