The torsion command sets the angle defined by the four specified atoms to angle in degrees, or if angle is omitted, reports the current value in the Log. A centroid or marker can be specified as an atom. For changing the angle, the two middle atoms must be covalently bonded to one another. The move option specifies whether to move the large or small side of the bond (default small, the side with fewer atoms). For dragging to rotate bonds interactively, see mousemode .
For measuring or changing valence angles defined by three atoms, see the angle command. See also: build, bond, turn, swapaa, altlocs, Angles/Torsions, measurements, making movies
For example, the following commands measure a ψ angle and set it to 120°, respectively:
torsion /a:11@n,ca,c:12@n 120