Theoretical calculations are reported for the interaction potential of Si-3 + NH3. The existence of loosely bound physisorbed states separated by sizable barriers from more tightly bound states more akin to dissociative chemisorption is predicted. These findings correspond to experimental results on this system which suggest ammonia desorbs intact from small silicon clusters, a behavior expected for physisorbed species. Qualitative explanations using frontier molecular orbital theory help explain the differences between the interaction of ammonia with small clusters and bulk silicon.