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[lammps-users] The infile section of fix rigid is somewhat wrong
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[lammps-users] The infile section of fix rigid is somewhat wrong

From: Sonu Kumar <sonubwbs@...8...>
Date: Sun, 17 Dec 2017 10:25:34 +0000

Dear users,
I am attaching a simplistic code considering two discs attached in 2d.
Two cases are considered. The first case has the rigid body's center lying at the origin and thus the principal axis matches with the simulation box. However, the second case has its center at x=-1. The manual for lammps suggests that the moment of inertia be given from the center of simulation box and the lammps computes its moment of inertia about rigid body's principal axes internally.

So I gave Izz=0.5891 for the case in which the origin lies at the center while Izz=2.1599 for the case in which origin lies at the x=-1. Now I used compute rigid/local to compute its Izz along the rigid body's principal axis(as mentioned in lammps manual). I am observing that lammps is taking Izz provided in the infile for fix rigid/small as its moment of inertia along the principal axis of the rigid body.

In the link you can check the Izz given in inp.txt (infile) and output tmp.dump and see both are giving the same value.

Sonu Kumar
Final Year B'Tech
Chemical Engineering, IIT Guwahati