distance_deriv.hh

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//
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/// @file   numeric/deriv/angle_deriv.hh
/// @brief  inline function for computing f1/f2 derivatives for a function of a distance
/// @author Phil Bradley did all the hard work deriving the math represented here.
/// @author Andrew Leaver-Fay copy-and-pasted Phil's code into this file from
/// the AtomPairConstraint.cc file for general use.

#ifndef INCLUDED_numeric_deriv_distance_deriv_hh
#define INCLUDED_numeric_deriv_distance_deriv_hh

#include <numeric/xyzVector.hh>

namespace numeric {
namespace deriv   {

/// @brief Compute the f1/f2 derivative vectors for point p1 for a function F of the
/// distance between p1 and p2.  This function returns the distance which should
/// be used to evaluate dF_ddist.  dF_ddist should then be multiplied into both
/// f1 and f2.  The values of the output variables f1 and f2 are overwritten.
template < class P >
inline
void
distance_f1_f2_deriv(
  xyzVector< P > const & p1,
  xyzVector< P > const & p2,
  P & distance,
  xyzVector< P > & f1,
  xyzVector< P > & f2
)
{
  typedef P Real;

  f2 = p1 - p2;
  distance = f2.length();
  if ( distance != Real(0.0) ) {
    Real const invd = Real(1.0) / distance;
    f1 = p1.cross( p2 );
    f1 *= invd;
    f2 *= invd;
  } else {
    f1 = Real(0.0);
  }
}


}
}

#endif