This is part of the vatom module

Calculate the center for a group of atoms, with arbitrary weights.

The computed center is stored as a virtual atom that can be accessed in an atom list through the label for the CENTER action that creates it. Notice that the generated virtual atom has charge equal to the sum of the charges and mass equal to the sum of the masses. If used with the MASS flag, then it provides a result identical to COM.

When running with periodic boundary conditions, the user should take care that the atoms in the COM group actually are in the proper periodic image. This is typically achieved using the WHOLEMOLECULES action before COM calculation.

The atoms involved can be specified using

ATOMS

the list of atoms which are involved the virtual atom's definition. For more information on how to specify lists of atoms see Groups and Virtual Atoms

Options

MASS

( default=off ) If set center is mass weighted

WEIGHTS

Center is computed as a weighted average.

Examples

# a point which is on the line connecting atoms 1 and 10, so that its distance
# from 10 is twice its distance from 1:
c1: CENTER ATOMS=1,1,10
# this is another way of stating the same:
c1bis: CENTER ATOMS=1,10 WEIGHTS=2,1

# center of mass among these atoms:
c2: CENTER ATOMS=2,3,4,5 MASS

d1: DISTANCE ATOMS=c1,c2

PRINT ARG=d1

(See also DISTANCE, COM and PRINT).