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

[Christoph Junghans]

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:
http://pubs.acs.org/doi/abs/10.1021/jp952646x

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.

Cheers,

Christoph


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