(ABCD)
is defined as the angle between the normals to the planes ABC
and BCD.
For a sequence of four atoms A, B, C, and D
the torsion angle
(ABCD)
is defined as the angle between the normals to the planes ABC
and BCD.
By convention is
positive if the sense
of rotation from BA to CD, viewed down BC, is clockwise,
otherwise it is negative.
Note that
(ABCD) and
(DCBA) have
the same sign; furthermore, the sign of a torsion angle does not change by
rotation or translation, and is reversed by
reflection or
inversion.
According to the the definition:
and then:
