How to calculate dihedral (not torsion) angles?

Andrew Martin amartin at
Thu Mar 18 04:35:47 EST 1999

Suhail Islam (islam at wrote:
: I hope I am not getting too confused here (!), but why
: can you not simply use the torsion angle calculation.
: Bonding does not matter, only the order of the 4 points
: which you consider. This will give you a correct sign.
: The sign is the sign of the volume formed by the
: 4 points. Angle between planes do not have a sign (handedness),
: becuse there are an infinite number of ways to order 4 points
: on the 2 planes.

I also do not understand the problem here :-)

you only need to look at any piece of code like Gromos Charmm/Xplor
to see calculation of improper dihedrals (which is what is being
asked for) is done by exactly the same piece of code as the proper
dihedral calculation (i.e. where the middle 2 atoms are bonded).

Best wishes,


Dr. Andrew C.R. Martin                              UCL/Inpharmatica 
UCL:          martin at 
Inpharmatica: a.martin at
Home:         andrew at

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