From: Alexei Podtelezhnikov (
Date: Thu Aug 04 2005 - 10:09:38 CDT

Hi vmd-l!

May I suggest the following optimization to dihedral angle calculations that relies on atan2f, rather than signed_angle. It's just faster and cleaner.

float dihedral(const float *a1,const float *a2,const float *a3,const float *a4)
  float r1[3], r2[3], r3[3], n1[3], n2[3];
  vec_sub(r1, a2, a1);
  vec_sub(r2, a3, a2);
  vec_sub(r3, a4, a3);

  cross_prod(n1, r1, r2);
  cross_prod(n2, r2, r3);

  float psin = dot_prod(n1, r3) * sqrtf(dot_prod(r2, r2));
  float pcos = dot_prod(n1, n2);

  return 57.2958f * atan2f(psin, pcos);

and we're done. I would drop signed_angle if nothing else uses it.

Thanks for vmd,
Alexei Podtelezhnikov, Ph.D.