center-of-mass

Dear Professor,
Now,I am using the lmp11_g++ , I want to output the center-of-mass of the group of atoms (rigid body). And in the manual of the lmp9_g++, I know the command of “fix com command” can calculate it .However, in the manual of the lmp11_g++, after reading the manual , I write the following two commands:
compute 3 tip com
dump 3 tip custom 100 dump.%.displace c_3
However there is an error : Dump custom compute does not compute per-atom info-catch_rsh /opt/mpich2/gnu.
I am puzzled with the problem . Could you give me some guidance , please.
Best regards ,
Liujing.

Dear Professor,
Now,I am using the lmp11_g++ , I want to output the center-of-mass of the
group of atoms (rigid body). And in the manual of the lmp9_g++, I know the
command of "fix com command" can calculate it .However, in the manual of
the lmp11_g++, after reading the manual , I write the following two
commands:
compute 3 tip com
dump 3 tip custom 100 dump.%.displace c_3
However there is an error : Dump custom compute does not compute per-atom
info-catch_rsh /opt/mpich2/gnu.
I am puzzled with the problem . Could you give me some guidance , please.

why is this confusing?

you instruct lammps to write out a "custom" dump,
which is used to output per-atom data, but then
try to output a quantity that is *not* computed on
a per atom basis, but computed globally.

on top of that, the error message tells you this
and the manual explains how to output global
and per atom quantities.

please unpuzzle yourself by studying this and the documentation
of the corresponding commands again and more carefully.

cheers,
      axel.