I am not familiar with the atom type 50, but there was a problem with inter- actions between 2 benzene rings using the MM2 force field: the attraction between benzenes in sandwich configuration was too strong comparing with the T-shaped dimer. For reference see: I.Pettersson & T.Liljefors Journal of Computational Chemistry 8 (1987) 1139 and the next paper in that issue by Allinger and a coworker. Slawomir Blonski