Sheldrick's 1.2 A rule and beyond.

An average profile of squared normalized structure factors as a function of resolution, /E/2>(d*), calculated from a large ensemble of high-resolution protein models, is presented. An interpretation is given that provides a structural explanation for Sheldrick's 1.2 A rule for the applicability of direct methods. The implications for the potential effectiveness of extended direct methods, incorporating stereochemical knowledge, are discussed.