HP-34C calculator. I t uses 78 program steps and 16 data registers. The input to the program consists of the observed torsion angles Boo. . . 8d0 and starting values of 8, and P. The output comprises the refined values of 8,, and P and various criteria of convergence and goodness-of-fit. One least-squares cycle takes 25 s. The program has been used successfully for a number of structures of ribonucleosides (42-441, all of which contain the five-membered ribofuranose ring. This exercise can be useful a t a junior, senior, or early graduate level. At the simplest level, the students are given the Bo,O . . 04Qtorsion angles derived from X-ray structure determinations and thev use the program to refine 8, and P, watching the convergen& criteria-of
