Dihedral angle

dihedral.jl

# Public Domain

using LinearAlgebra: cross, dot, norm

"""
    dihedral(r1, r2, r3, r4)

Compute the dihedral angle of two planes defined by (`r1`, `r2`, `r3`) and
(`r2`, `r3`, `r4`).

Reference: https://en.wikipedia.org/wiki/Dihedral_angle
"""
function dihedral(r1, r2, r3, r4)
    u1 = r2 - r1
    u2 = r3 - r2
    u3 = r4 - r3
    x = cross(u1, u2)
    y = cross(u2, u3)
    return atan(norm(u2) * dot(u1, y), dot(x, y))
end