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Re: [ESPResSo-devel] Pressure calculation / P3M stress tensor
From: |
Markus Deserno |
Subject: |
Re: [ESPResSo-devel] Pressure calculation / P3M stress tensor |
Date: |
Fri, 03 Apr 2009 13:46:12 -0400 |
User-agent: |
Thunderbird 2.0.0.6 (X11/20070801) |
Hi all,
> as far as I can see it is not obsolete. In line 175 1/3 of the total
> pressure is put on the diagonal of pressure tensor. This helps to make
> the trace of the pressure tensor, which is the pressure itself,
> correct. But all off-diagonal coulomb terms are 0. So e.g.
> calculations of viscosity out of the pressure tensor will be wrong.
>
I suspected that this has not yet been done. Someone needs to sit down
and concentrate real hard and code it...
Moreover: The current solution seems wrong to me too, even in the isotropic
case. Here's why:
The pressure is (1/3) times the trace of the stress tensor.
What the current code seems to assume (just like previous version
of the non-coulomb-stresses) is that the pressure is the trace of the
stress tensor, and hence the individual components are 1/3 of the
isotropic ones. This is wrong.
So, for a system with isotropic pressure P and no off-diagonal
stresses the stress tensor is
( P 0 0 )
( 0 P 0 )
( 0 0 P )
(up to possibly a sign that I can't remember now.)
Please be aware of that. One of my papers had a stupid factor
of 3 mistake because of that, and later in a different research project
it took me and Vagelis weeks to figure out that our research was
right but the stress tensor was a factor of 3 too small...
And: I fear all of that happened because the respective equation
is wrong in my PhD thesis, which might be the source of all that pain...
Best,
Markus
--
Dr. Markus Deserno
Associate Professor of Physics ++1-412-268-4401 (office)
Carnegie Mellon University ++1-412-681-0648 (fax)
5000 Forbes Avenue ++1-412-268-8367 (Donna Thomas)
Pittsburgh, PA 15213 address@hidden