Re: [ESPResSo-users] time dependent force

[Ulf Schiller]

Jiajia Zhou wrote:
> I mean if I use "integrate 1", then the forces have to be calculated
> before the first step of vv scheme and also at the third step.

This is true for the first call to "integrate". In successive calls, the 
forces in the first step are the same as in the third step of the 
previous integration. So unless you make changes to your system that 
affect the forces between calls to integrate, the forces will be reused 
and not recalculated.

Nevertheless, there lies the reason why you _have_ to implement a time 
dependent external force on the C-level, because the two occurrences of 
the forces  within the integration are forces at different times - f(t) 
and f(t+dt), respectively. Therefore, setting the force at the TCL level 
will not give you the correct result.

Best regards,
Ulf

> The potential I used is just a sinusoidal potential. I am running some
> tests right now. If they are fine then I will be happy to send the
> relative codes.
>
> Thanks for your help!
> Jiajia
>
> On Wed, Jul 20, 2011 at 5:35 PM, Axel Arnold
> <address@hidden> wrote:
> > On Wednesday 20 July 2011 16:02:41 Jiajia Zhou wrote:
> > > Thanks for the reply. I would like to change the force every single
> > > time step. To do this in the tcl level, I think the force are
> > > calculated twice for every time step (am I correct?), so I can save
> > > some time by implementing in the c level.
> > Hi,
> >
> > no, the forces are normally calculated only once per time step, but twice used
> > in the propagation. Nevertheless, if you have e.g. a periodically oscillating
> > force, you definitely want to implement it as an constraint.
> >
> > Btw, if your external potential is not very exotic, such as e.g. a simple
> > sinusoidal potential sin(kx + wt) or so, it would be nice if you could send us
> > the code for integration with the next release, since that might be of general
> > interest.
> >
> > Axel
> >
> > --
> > JP Dr. Axel Arnold
> > ICP, Universität Stuttgart
> > Pfaffenwaldring 27
> > 70569 Stuttgart, Germany
> > Email: address@hidden
> > Tel: +49 711 685 67609

--
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Institute of Complex Systems (ICS-2)       Phone:   +49 2461 61-6144
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