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Re: [ESPResSo-users] time dependent force

From: Ulf Schiller
Subject: Re: [ESPResSo-users] time dependent force
Date: Thu, 21 Jul 2011 08:16:03 +0200
User-agent: Thunderbird (X11/20071031)

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,

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!

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.

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


JP Dr. Axel Arnold
ICP, Universit├Ąt Stuttgart
Pfaffenwaldring 27
70569 Stuttgart, Germany
Email: address@hidden
Tel: +49 711 685 67609

Dr. Ulf D. Schiller                        Building 04.16, Room 3006
Institute of Complex Systems (ICS-2)       Phone:   +49 2461 61-6144
Forschungszentrum J├╝lich, Germany          Fax:     +49 2461 61-2850

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