CCL:G: Fixing distance between point charge and atoms

 Sent to CCL by: "Chunyi  Sung" [juneyi1(~)yahoo.com.tw]
 Hi, CCLers
       I am trying to optimize some molecules in the presence of point charges
 using the keyword "charge" in Gaussian; however, in my case, some O
 atom in the molecule always ended up colliding with a positive point charge
 during geometry optimization. Therefore, I want to somehow set up some
 constraints on that O atom so that it will stay, say, 2.7 Angstrom away
 > from the positive point charge, does anyone know how to do that ?
       Originally, I wanted to put a ghost atom right at the location of the
 positive point charge, and then use "modredundant" to fix the bond
 length between the O atom and the ghost atom. However, with the
 "charge" keyword to include point charges, "Opt" must have
 "Z-matrix", which does not work with "modredundant". So I am
 now running out of ideas. I'll appreaciate if
 anybody has any suggestions on this.
 thanks
 chunyi