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Re: [ESPResSo] DPD and velocity verlet integration
From: |
Axel Arnold |
Subject: |
Re: [ESPResSo] DPD and velocity verlet integration |
Date: |
Tue, 28 Jul 2009 10:57:14 +0200 |
User-agent: |
KMail/1.9.4 |
On Saturday 25 July 2009 23:03, Jacob Kirkensgaard wrote:
> Hello
>
> I am implementing some DPD polymer simulations and would like to use
> the modified velocity-Verlet (VV) algorith used by Groot and Warren in
> Groot and Warren, J. Chem. Phys. 107 (11) 1997. I attach a snapshot of
> the equations:
> As is stated the usual VV is recovered for lambda = 0.5. The
> documentation is close to non-existing on this matter so here are the
> questions:
>
> 1: Does the implemented version of the VV include the 0.5*Delta t^2
> term in the first step? This is shown in the above paper to be crucial
> to temperature control in DPD. There is mentioning of some scaling of
> the forces in the code (integrate.c) but I am unsure if it means
> exactly this...
In Espresso, the forces are rescaled by 0.5*Delta t^2, and the velocities by
Delta t. Therefore, for example your first equation reduces to:
r(t+Delta t) = r(t) + v(t) + f(t)
> 2: I suspect the lambda thing is NOT implemented in Espressos version
> of the VV - does anyone know?
No, it isn't, but on the other hand, should be easy to implement. The second
equation in the Espresso rescaling reads:
tilde v(t + Delta t) = v(t) + 2 lambda f(t)
Since Espresso so far does only lambda=1/2, there is no prefactor in front of
f(t), but that wouldn't be difficult to introduce. The places to look into
are propagate_vel_pos and propagate_vel. There are two routines, since the
combining of step one and two of the Verlet scheme safes some time, but
cannot be done for some algorithms.
Cheers,
Axel
--
Dr. Axel Arnold
Fraunhofer SCAI
Schloss Birlinghoven, 53754 Sankt Augustin, Germany
Tel: +49 2241 14 2575