For bond angles, 
is the angle defined by three atoms, labeled
i, j, k, with j being the central atom.
Let 
be the vector from atom j to atom i, 
the vector
from atom k to atom j and 
the vector from atom i to atom k.
Then
[Note: In Fortran the argument of the function ACOS is in radians. If 
is to be in degrees a conversion factor must be included.]
where:
