Help on force field for simplified "atoms"

Dear folks,
 Does any of you have the experience on developing force field for
 simplified "atoms"?
 I know there are some papers related with simplified "atom" for CH2
 or CH3, but how about biger molecules?
 For example, if I want to get a simple function V(R) for the interction
 between C6H6 and C6H6, I may either summarize the LJ(rij) interctions
 between all of atoms(i in one molecule and j in the other one), or I can
 do ab initio calculations for different intermolecular distances(R). By
 both methods I may be able to fit the results into a simple function, say
 still in LJ(R) form.  However, sometimes this mayn't work, or very
 time-consuming for big molecules.
 If anyone could give me some suggestions or related reference, I will be
 very grateful.
 Thanks a lot!
 Guosheng Wu
 Guosheng Wu
 Dept. of Chem.,  Northwestern Univ.
 2145 Sheridan Road, Evanston, IL, 60208-3113