compute ID group-ID com
compute 1 all com
Define a computation that calculates the center-of-mass of the group of atoms, including all effects due to atoms passing thru periodic boundaries.
A vector of three quantites is calculated by this compute, which are the x,y,z coordinates of the center of mass.
IMPORTANT NOTE: The coordinates of an atom contribute to the center-of-mass in "unwrapped" form, by using the image flags associated with each atom. See the dump custom command for a discussion of "unwrapped" coordinates. See the Atoms section of the read_data command for a discussion of image flags and how they are set for each atom. You can reset the image flags (e.g. to 0) before invoking this compute by using the set image command.
IMPORTANT NOTE: If an atom is part of a rigid body (see the fix rigid command), it's periodic image flags are altered, and its contribution to the center-of-mass may not reflect its true contribution. See the fix rigid command for details. Thus, to compute the center-of-mass of rigid bodies as they cross periodic boundaries, you will need to post-process a dump file containing coordinates of the atoms in the bodies.
This compute calculates a global vector of length 3, which can be accessed by indices 1-3 by any command that uses global vector values from a compute as input. See this section for an overview of LAMMPS output options.
The vector values are "intensive". The vector values will be in distance units.