For torsion angles, 
is the angle defined by the four atoms
labeled i, j, k, l. The quantity is the angle between the normal to
the plane defined by atoms i, j, k and the normal to the plane defined
by atoms j, k, l.
Let 
be the vector from atom j to atom i, 
the
vector from atom j to atom k and 
the vector from atom k to
atom l. Also let 
and
where 
is the normal to
the plane containing atoms i, j, k and 
is the normal to the
plane containing atoms j, k, l.
[Note: In Fortran the argument of the function ACOS is in radians. If
is to be in degrees a conversion factor must be included.]
Let
and
Then
