A correction in my last message about special position
> 1. If the atoms at the special positions are mostly rigid,
> just fix them by using
> CONSTRAINTS FIX=(special-atoms) END
> which turns off all interactions for the special atoms.
The "CONSTRAINTS FIX" alone does not turns off interactions.
One still has to use
INTERACTION ( NOT (special-atoms) ) ( NOT (special-atoms) )
to exclude self interactions for the fixed special atoms.