Re: [ESPResSo-users] Force scaling wrong

[Ulf Schiller]

On 02/28/2015 12:09 PM, Joost de Graaf wrote:
> Dear Floh & Marcello,
> 
> I'm not sure if it's that good an idea to make the forces read-only. I
> can assure you, they are definitely not that way in the current code and
> for good reasons. If you want to reinitialize the LB fluid, you need the
> reuse_forces command, to account for the fact that the LB use of forces
> is half a time step off of that in the main integration loop. That's why
> you want to keep them settable. (as I wrote this Henri's message came in).

This is only needed if you desire exactly reproducible trajectories
(which of course is helpful for debugging). The sampling would still be
valid (and actually more accurate) if one just recalculates the forces
(adjusting the noise amplitude appropriately). That is basically just
another instance where velocity dependent forces reduce the accuracy of
ESPResSo's integrator by an order anyways, same for the LB update.

Cheers,
Ulf

> On 28 February 2015 at 12:23, Florian Weik <address@hidden
> <mailto:address@hidden>> wrote:
> 
>     Hi,
>     Marcello is right, these forces are never used. For technical
>     reasons it is not that easy to make them read-only because a lot of
>     the test cases rely on the fact that forces can be read via the
>     blockfile mechanism which is in turn just an interface to the tcl
>     part command. So unless somebody is volunteering for the busywork to
>     change dozens of test cases, that situation is staying the way it
>     is. I'm pretty sure the documentation reflects the fact that these
>     forces are not used.
> 
>     Cheers,
>     Florian
> 
>     On Sat, Feb 28, 2015 at 8:36 AM, Marcello Sega
>     <address@hidden <mailto:address@hidden>> wrote:
> 
>         Hi,
> 
>         please correct me if I'm wrong: I think that forces are always
>         overwritten by the integrator, so that setting them by hand does not
>         have any effect at all.
> 
>         If this is right, then allowing to set forces could be just
>         misleading
>         (as the program would behave in a way unexpected by the user).
>         Wouldn't it be better to make them read-only?
> 
>         Regards,
> 
>         Marcello
> 
> 
> 
> 
> 
> 
>         On Fri, Feb 27, 2015 at 3:56 PM, Henri Menke
>         <address@hidden <mailto:address@hidden>>
>         wrote:
>         > Dear all,
>         >
>         > in the current master of ESPResSo the forces are scaled by the
>         masses
>         > and the time step in the output, i.e. part 0 print f, but are
>         not scaled
>         > by the mass in the input, i.e. part 0 f.  This leads to wrong
>         forces
>         > when setting them by hand with a mass not equal to 1.
>         >
>         > This bug is fixed in the pull-request
>         >
>         >   https://github.com/espressomd/espresso/pull/213
>         >
>         > Kind regards,
>         > Henri
>         >
> 
> 
> 
>         --
>         Institut für Computergestützte Biologische Chemie
>         University of Vienna
>         PGP public key on MIT server http://goo.gl/zlIZix
> 
> 
> 
> 
>     -- 
>     Florian Weik, Dipl.-Phys.,
>     Institut für Computerphysik, Allmandring 3, D-70569 Stuttgart
>     Phone: +49-711-685-67703 <tel:%2B49-711-685-67703>
>     Public Key 0x0562F11D Fingerprint 3294 6360 EC93 37A3 BD40  F597
>     0BAD 3AF8 0562 F11D
> 
> 


-- 
Ulf D. Schiller
Centre for Computational Science
University College London
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