Torsion.h

#ifndef SIMP_TORSION_H
#define SIMP_TORSION_H

/*
* Simpatico - Simulation Package for Polymeric and Molecular Liquids
*
* Copyright 2010 - 2017, The Regents of the University of Minnesota
* Distributed under the terms of the GNU General Public License.
*/

#include <util/space/Vector.h>

#include <cmath>

namespace Simp
{

   using namespace Util;

   struct Torsion
   {
   
   public:

      double cosPhi;

      bool computeAngle(const Vector& b1, const Vector& b2, const Vector& b3);
 
      double sinPhi() const;

      double phi() const;

   };

   // Inline method definitions

   /* 
   * Calculate cosPhi.
   */ 
   inline bool
   Torsion::computeAngle(const Vector& b1, const Vector& b2, const Vector& b3)
   {
      Vector v1, v2;
      double d1, d2;

      v1.cross(b1, b2);
      d1 = v1.square();
      if (d1 < 1.0E-10) {
         return 1;
      }
      d1 = sqrt(d1); 

      v2.cross(b2, b3);
      d2 = v2.square();
      if (d2 < 1.0E-10) {
         return 1; // error code
      }
      d2 = sqrt(d2); 

      cosPhi = v1.dot(v2)/(d1*d2);
      return 0; // Normal completion
   }

   /*
   * Return value sin(phi) for precomputed cos(phi).
   */
   inline
   double Torsion::sinPhi() const
   {  return sqrt(1.0 - cosPhi*cosPhi); }

   /*
   * Return phi in radians for precomputed cos(phi).
   */
   inline
   double Torsion::phi() const
   {  return std::acos(cosPhi); }

} 
#endif