[gmx-users] dihedral angle

Mark Abraham Mark.Abraham at anu.edu.au
Wed Nov 16 05:05:25 CET 2011

On 16/11/2011 5:46 AM, Nilesh Dhumal wrote:
> Hello,
> I am trying to calculate the dihedral angle between four points.
> I considered two planes (ABC&  BCD) and calculate the dot products of two
> normals (n1 and n2)to these plane.
> theta=cos-1(n1.n2)
> I compared theeta values of dihedral angle with ampac and some values are
> negative. I am geting all positive value.
> Any one knows how to calculate sign of dihedral angle.

The sign of the angle is determined by the order in which the atoms 
define the plane. Conventionally, the normal of ABC is in the opposite 
direction of the normal of CBA, and the two angles those normals form 
with the normal of BCD have equal magnitude but opposite sign of their 


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