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Re: [ESPResSo-devel] Local stress tensor/Multibody interaction and A bug

From: Christoph Junghans
Subject: Re: [ESPResSo-devel] Local stress tensor/Multibody interaction and A bug in tensor distribute
Date: Tue, 16 Jun 2009 22:57:03 +0200
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Hi all again,

The pressure might not depend on them globally (but I'm not fully
certain here), but it surely matters locally.  So whenever one wants a
spatially resolved stress tensor, these terms matter.  Since many
people now use ESPResSo with more than just two-body-potentials, none
of them can use the local stress tensor.  Are there efforts on the way
to fix this?  (No point to start doing this when somebody else has
already done so).
You are totally right it matters for any local pressure quantity and it also matters for the stress tensor. Only the total pressure (a number) is unchanged, (at which I am looking most of the time). I did not remember the reference now, but I guess it was this one:

I think it is not too much effort to change it. add_bonded_virials is the function to change.

It will depend very much on the system you study how reliable the
results are without these contributions.

Strange answer... If you're sure that the multibody terms don't affect
the global pressure, then the answer should be that there will be no
effect whatsoever from leaving them out.  Maybe you're not sure?  For
the local stresses your answer might be true, though.
You are right, I did not make my point very clear. It matters for the stress tensor in any case, but the question is how big are the forces from the angle potential in comparison to the 2-body forces. Maybe this contribution is just small and negligible (but this will depend on the system studied).

Sure it's a feature: It's a security check that tests whether the
distribution of the virial between the boxes is done completely
(without bits missing).  In our case the error can become as big as
1e-8, which is way too large, a million times bigger than
ROUND_ERROR_PREC.  Something clearly seems to be wrong here, yet I
would not know why this should depend on the system.  Seems like we
need to go through the local stress tensor routine and check it...
I guess in the worst case it could be:
(number of addend in loop)*ROUND_ERROR_PREC
We should take a look at size of the boxes. Btw. why is not fabs(length-calclength) in condition of the error message.



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