When I try to do a minimization I get a message that indicates that parameters are missing, in particular, torsional parameters.
For many parameters, such as torsional angles, there are literally millions that are needed to fit all possible cases. There are simply too many to provide in Chem3D.
The latest version of Chem3D will make a guess at missing parameters rather than stopping the minimization as occurs in versions before 3.2. In versions before 3.2, you can get a good estimate by substituting certain parameters that are missing with replacement values suggested in the paper sited below, realizing of course, that the result will be a guess.
"Development of an Internal Searching Algorithm for Parameterization of the MM2/MM3 Force Fields" Dora M. Schnur. Mark V. Grieshaber, and J. Phillip Bowen Journal of Computational Chemistry, Vol. 12, No. 7, 844-849 (1991).