Improper TorsionΒΆ
Commonly used in the AMBER force fields, it defines a hypothetical torsional angle for four atoms not successively bonded, e.g., a trigonal center. The functional form is similar to the proper torsion,
\[U = \sum_{n=1}^3 V_n[1+\cos(n\phi-\delta_n)],\]
where n is the periodicity and \(\delta_n\) is the corresponding phase shift.