By use of the potential model of the molecular mechanics for clusters (MMC) scheme, calculations have been performed on about 100 weakly bound trimers that consist of at least 2 like molecules. The results provide insight for the spectroscopic investigation of several trimers, including comparative information on the energetics of trimer formation and estimates of equilibrium rotational constants. The results also provide a basis for understanding the importance of various potential terms in the overall interaction energetics. Hydrogen bonding emerges as somewhat distinct from general weak bonding because of the typically strong electrical interaction when there is an intervening hydrogen center. A remarkable tendency of water to bind to both polar and nonpolar species develops as a consequence. There are two areas of application for weak interaction potential schemes : the detailed connection with small cluster spectroscopy and the simulation of large and complex systems. From this survey of clusters, the most crucial elements for the different applications can be recognized.