The TWIST command rewrites the PSF used in the analysis facility in order to create virtual dihedrals spanning the alpha and beta carbons in the backbone of a protein. For residue i, the dihedrals formed are CB(i) — CA(i) — CA(i-1) — CB(i-1). If a residue is a glycine, then no entry is made for it. The old list of dihedrals is destroyed (so be forewarned!), but only in the analysis facility. The PSF used by the top level of CONGEN is still intact so exiting the analysis facility and reentering it will restore the dihedrals list.
The COMPARE option controls which analysis PSF is used. When this keyword is omitted, then the main PSF is used; otherwise, the comparison PSF is used.
Once this command has been executed, then the twists can be obtained by a BUILD TORSION GEOMETRY command, and subsequent processing of the table by the analysis facility.