Torsion.cpp

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/* +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
   Copyright (c) 2013 The plumed team
   (see the PEOPLE file at the root of the distribution for a list of names)

   See http://www.plumed-code.org for more information.

   This file is part of plumed, version 2.0.

   plumed is free software: you can redistribute it and/or modify
   it under the terms of the GNU Lesser General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   plumed is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public License
   along with plumed.  If not, see <http://www.gnu.org/licenses/>.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ */
#include "Torsion.h"
#include "Tensor.h"

#include <cmath>
#include <iostream>

namespace PLMD{

double Torsion::compute(const Vector& v1,const Vector& v2,const Vector& v3)const{
    const Vector nv2(v2*(1.0/v2.modulo()));
    const Vector a(crossProduct(nv2,v1));
    const Vector b(crossProduct(v3,nv2));
    const double cosangle=dotProduct(a,b);
    const double sinangle=dotProduct(crossProduct(a,b),nv2);
    return std::atan2(-sinangle,cosangle);
}

double Torsion::compute(const Vector& v1,const Vector& v2,const Vector& v3,Vector& d1,Vector& d2,Vector& d3)const{

  d1.zero();
  d2.zero();
  d3.zero();

  const double modv2(v2.modulo());
  const Vector nv2(v2/modv2);
  const Tensor dnv2_v2((Tensor::identity()-extProduct(nv2,nv2))/modv2);

  const Vector a(crossProduct(v2,v1));
  const Tensor da_dv2(dcrossDv1(v2,v1));
  const Tensor da_dv1(dcrossDv2(v2,v1));
  const Vector b(crossProduct(v3,v2));
  const Tensor db_dv3(dcrossDv1(v3,v2));
  const Tensor db_dv2(dcrossDv2(v3,v2));
  const double cosangle=dotProduct(a,b);
  const Vector dcosangle_dv1=matmul(b,da_dv1);
  const Vector dcosangle_dv2=matmul(b,da_dv2) + matmul(a,db_dv2);
  const Vector dcosangle_dv3=matmul(a,db_dv3);

  const Vector cab(crossProduct(a,b));
  const Tensor dcab_dv1(matmul(dcrossDv1(a,b),da_dv1));
  const Tensor dcab_dv2(matmul(dcrossDv1(a,b),da_dv2) + matmul(dcrossDv2(a,b),db_dv2));
  const Tensor dcab_dv3(matmul(dcrossDv2(a,b),db_dv3));

  const double sinangle=dotProduct(cab,nv2);
  const Vector dsinangle_dv1=matmul(nv2,dcab_dv1);
  const Vector dsinangle_dv2=matmul(nv2,dcab_dv2)+matmul(cab,dnv2_v2);
  const Vector dsinangle_dv3=matmul(nv2,dcab_dv3);

  const double x=-sinangle/cosangle;
  const Vector dx_dv1=-dsinangle_dv1/cosangle + sinangle/(cosangle*cosangle) * dcosangle_dv1;
  const Vector dx_dv2=-dsinangle_dv2/cosangle + sinangle/(cosangle*cosangle) * dcosangle_dv2;
  const Vector dx_dv3=-dsinangle_dv3/cosangle + sinangle/(cosangle*cosangle) * dcosangle_dv3;

  const double torsion=std::atan2(-sinangle,cosangle);
  d1=1.0/(1.0+x*x) * dx_dv1;
  d2=1.0/(1.0+x*x) * dx_dv2;
  d3=1.0/(1.0+x*x) * dx_dv3;
  
  return torsion;
}

}