The itor improper torsion energy is the sum of N_imp terms where each term has the form:

kimp ( tijkl - to )2   

where k_imp is a force constant, t_ijkl is the current torsion angle spanned by atoms atom_i, atom_j, atom_k and atom_l and t_o is the equilibrium torsion angle. The torsion angle is calculated as a signed quantity so t_o can range from -3.142 radians (-180 degrees) to +3.142 radians (+180 degrees). The itor record in the descriptor has the form:

itor	Nimp
	atomi	atomj	atomk	atoml	kimp	to
	...	...	...	...	...	...

The itor record starts with the keyword itor followed by the number of improper torsions N_imp, then N_imp lines of six fields each: the four atoms that span a torsion angle atom_i, atom_j, atom_k and atom_l, the force constant k_imp and the equilibrium value of the torsion angle t_o. The sign of the torsion is significant here. For example, the torsion spanned by atoms (1,2,3,4) has the same magnitude but is opposite in sign to the torsion spanned by (4,3,2,1).

Example:

itor   3

1079   1080   3   2   45461.7   0.6283
   2      3   6   5   45461.7   0.6283
   5      6   9   8   45461.7  -0.5236

An example of three improper torsion angles, the equilibrium angle is 0.6283 radians or 36 degrees for the first two and -0.5236 radians or -30 degrees for the last angle.