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Re: [lammps-users] Virial part in Pressure calculation

Re: [lammps-users] Virial part in Pressure calculation

 From: Axel Kohlmeyer Date: Fri, 13 Oct 2017 08:46:25 -0400

```On Fri, Oct 13, 2017 at 4:33 AM, Xiao Jia <xiaoj@...1508...> wrote:
> Dear LAMMPS users,
>
> Based on the description of compute_pressure, the virial part will be the
> summation of Fi_dot_ri, where Fi is the net force on atom i and ri is the
> location of atom i.
> Based on this description, I have a question about how ri is defined.
> Is ri the absolute location of atom i or there is a reference origin?

r_i is the absolute location.

> If ri is the absolute location of atom i, based on the definition, will the
> pressure change if we have two systems at same status (assuming same Fi for
> atoms) but with different spatial location (translational offset)?

no. please recall, that all force contributions can be decomposed into
pairwise contributions.
for each pair ij, the virial contribution becomes: F_ij dot r_i +
-F_ij dot r_j = F_ij dot (r_i - r_j). and (r_i - r_j) is invariant to
translation of the entire system.

axel.

>
> Thanks!
>
>
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--
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.

```