ON THE DEFINITION AND USAGE OF TORSIONAL AND TWISTING COORDINATES IN FORCE-FIELD CALCULATIONS

Because different authors construct composite torsional coordinates differently, torsional force constant values reported for the same or very similar internal rotors differ from each other by as much as one or occasionally two orders of magnitude. Since these coordinates are not always precisely defined, the source of the discrepancies cannot always be delineated. Examples are given which show how the recommended torsional coordinate (IUPAC Commission on Molecular Spectroscopy) ensures that the G and F matrix values for the same internal rotor in different surroundings stay comparable. The other types of torsional coordinates which are frequently used lead to inconsistent results even with the same molecule. A similar problem can also exist for the twisting coordinate.