The iton torsion power series function is used to express unusual functional forms in torsion angle. There are N_iton terms of the form:

kn tijkln, if tl <= tijkl < th
   0, elsewhere
   

where t_ijkl is the current angle spanned by the atoms atom_i, atom_j, atom_k and atom_l; k_n is a constant and n is zero or a positive integer. The term is evaluated only if the torsion angle exceeds a lower limit t_l and is less than an upper limit t_h. The order of the atom number is significant, for example the torsion angle spanned by atoms (1,2,3,4) is not the same as that spanned by (2,1,3,4) although both angles are defined over the same four atoms. The torsion angle is evaluated as a signed quantity, for example the torsion angle spanned by the atoms (1,2,3,4) has the same magnitude as the angle spanned by (4,3,2,1) but is opposite in the sign. The torsion angle is calculated as a signed quantity, something to consider when constructing a power series that includes odd powers. The iton record has the form

iton	Niton
	atomi	atomj	atomk	atoml	kn	n	tl	th
	...	...	...	...	...	...	...	...

The torsion power series record starts with the keyword iton followed by the number of terms N_iton, then N_iton lines of eight fields each: the atoms atom_i, atom_j, atom_k and atom_l, the force constant k_n, the power n, the lower limit t_l and the upper limit t_h. For example

iton   3

1   3   6   4     2198.7   0   -3.142   3.142
1   3   6   4     1399.8   1   -3.142   3.142
1   3   6   4   -13144     2   -3.142   3.142
1   3   6   4    -4254.7   3   -3.142   3.142
1   3   6   4    20315     4   -3.142   3.142

A fourth-power series defined over atoms (1,3,6,4), which is a potential with two minima, at -30 degrees and at +36 degrees. The barrier between the two minima is at 5 kcal/mol. The range of the function is from slightly less than -180 degrees to slightly more than 180 degees, thus ensuring that the series is applied at all torsion angles.