Conformational analysis of nucleosides

Hi All!
 My question to you fellow CClers is as follows:
 I need to analyze (computationally) the conformations of various
 nucleosides, and ribonucleosides having modifications in the furanose and
 the heterocyclic base regions. Are you aware of any commercial and/ or
 shareware package that can achieve this. I will need to calculate atomic
 charges, assign proper atom properties and then perform molecular mechanics
 and dynamics simulations to assign determine the likely conformation of the
 molecules under study. A program that has the requisite force fields ( any
 suggestions for the proper force field to use would be appreciated), and
 should have reasonable cost and hardware requirements ( PC compatibility
 would be best).
 I look forward to the usual helpful responses and thank all respondents in
 Prasad Dande
 Prasad Dande, Ph. D.
 Senior Scientist
 Research Chemistry
 ISIS Pharmaceuticals
 2292 Faraday Avenue
 Carlsbad, CA 92008
 Tel: 760 603 4671
 Fax: 760 603 4654
 Prasad_Dande (- at -)