rephrase to use more common term

For the 'distance', 'angle', and 'dihedral' constraints (explained

above), atom IDs can be replaced by pre-reaction molecule-fragment

IDs. The molecule-fragment ID must begin with a letter. The location

of the ID is the unweighted average of all atom positions in the

of the ID is the geometric center of all atom positions in the

fragment. The molecule fragment must have been defined in the

:doc:`molecule <molecule>` command for the pre-reaction template.