/*
 * PairPotential_Harmonic.hpp
 *
 *  Created on: Sep 26, 2012
 *      Author: heber
 */

#ifndef PAIRPOTENTIAL_HARMONIC_HPP_
#define PAIRPOTENTIAL_HARMONIC_HPP_


// include config.h
#ifdef HAVE_CONFIG_H
#include <config.h>
#endif


#include "Potentials/EmpiricalPotential.hpp"
#include "FunctionApproximation/FunctionModel.hpp"

/** This is the implementation of a harmonic pair potential.
 *
 * This evaluates \f$ k \cdot (r -r_0)^2 \f$.
 *
 */
class PairPotential_Harmonic : public EmpiricalPotential, public FunctionModel
{
public:
  PairPotential_Harmonic();
  PairPotential_Harmonic(
      const double _spring_constant,
      const double _equilibrium_distance) :
    spring_constant(_spring_constant),
    equilibrium_distance(_equilibrium_distance)
  {}
  virtual ~PairPotential_Harmonic() {}

  /** Setter for parameters as required by FunctionModel interface.
   *
   * \param _params given set of parameters
   */
  void setParameters(const parameters_t &_params);

  /** Getter for parameters as required by FunctionModel interface.
   *
   * \return set of parameters
   */
  parameters_t getParameters() const;

  /** Getter for the number of parameters of this model function.
   *
   * \return number of parameters
   */
  size_t getParameterDimension() const
  {
    return 2;
  }

  /** Evaluates the harmonic potential function for the given arguments.
   *
   * @param arguments single distance
   * @return value of the potential function
   */
  results_t operator()(const arguments_t &arguments) const;

  /** Evaluates the derivative of the potential function.
   *
   * @param arguments single distance
   * @return vector with derivative with respect to the input degrees of freedom
   */
  derivative_components_t derivative(const arguments_t &arguments) const;

private:
  //!> the spring constant for the harmonic potential
  double spring_constant;
  //!> the equilibrium distance indicating the potential minimum
  double equilibrium_distance;
};

#endif /* PAIRPOTENTIAL_HARMONIC_HPP_ */