The critical_points feature is used to focus the orientation search into a subsite of the receptor active site (DesJarlais, et al. J. Comput-Aided Molec. Design. 1994 and Miller, et al J. Comput. Aided Mol. Design. 1994). For example, identifying molecules that interact with the catalytic residues might be of chief interest. Any number of points may be identified as critical (see Critical Points information on labeling spheres), and any number of groupings of these points may be identified. An alternative to using critical points is to discard all site points that are some distance away from the subsite of interest, while retaining enough site points to define unique ligand orientations. This feature can be highly effective at reducing matching by five-fold or more. It is particularly useful to also assign chemical labels to the critical points to further focus sampling.