[Pw_forum] Initial velocities for MD run? (Norbert Nemec)

[Norbert Nemec]

-------- Original-Nachricht --------
> Datum: Tue, 13 Jul 2010 10:44:09 +0100
> Von: Davide Ceresoli <davide.ceresoli at materials.ox.ac.uk>
>
> The CPV code has an option to read atomic velocities from input.
> However, it has never been clear to me in which units one should
> enter the velocities.

Thanks for the hint. I'll have a look at it.

As for the units: I would go for physical units that are independent of the MD timestep.

B.t.w: May it be that "zero initial velocity" is implemented incorrectly in the code? At the moment, the code sets 'tau_old=tau'. In Verlet, however, the current velocity is defined as '(tau_new-tau_old)/(2*dt)'.

To correctly set an initial velocity v0 in Verlet, the first time step has to be defined by the two conditions:

  tau_new+tau_old = 2*tau + a*dt**2
  tau_new-tau_old = v0*2*dt

eliminating tau_old and getting

  tau_new = tau+a/2*dt**2+v0*dt

Once the first two time steps are known, the algorithm can then continue without velocities as usual.
-- 
GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT!
Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01