about MM3 force field

From: jz7$at$duke.edu
Subject: about MM3 force field
Date: Wed, 6 Jul 2005 18:47:50 -0400 (EDT)

Dear all,
 I am running calculation (MD) on 1,8-diphenyl-naphthalene and similiar but
 longer structure using MM3 force field. Because it's a big conjugated
 system, I assigned all the carbon atoms to be pi-atom. However, the rings
 are tended to be in a plane for longer diphenyl-naphthalene molecules
 (1,8-diphenyl-naphthalene is ok). From the X-ray/NMD experiment, the
 phenyl rings should be almost perpendicular to the naphthalene. Would
 anyone please tell me whether my treatment is wrong or what else force
 field is better for this kind of molecule (for MD simulation)?
 Thanks a lot!