Enumerations for _Force_constant.Units (An open enumeration)
kcal/mol*A2	'bonds; distance; radius of gyration; planarity'
kcal/mol*A4	van der Waals
kcal/mol*Hz2	dipolar coupling; scalar couplings; T1/T2; CSA
kcal/mol*ppm2	Chemical shifts; pseudocontact shifts
kcal/mol*rad2	'angle or dihedral angle'