/home/js850/projects/pele/source/pele/base_potential.h

#ifndef PYGMIN_POTENTIAL_H
#define PYGMIN_POTENTIAL_H

#include <assert.h>
#include <vector>
#include <stdexcept>
#include <iostream>
#include "array.h"

namespace pele {

/***
 * basic potential interface for native potentials
 */
class BasePotential {
public:
    virtual ~BasePotential() {}

    virtual double get_energy(Array<double> x)
    {
        throw std::runtime_error("BasePotential::get_energy must be overloaded");
    }

    virtual double add_energy_gradient(Array<double> x, Array<double> grad)
    {
        throw std::runtime_error("BasePotential::add_energy_gradient must be overloaded");
    }

    virtual double get_energy_gradient(Array<double> x, Array<double> grad)
    {
        double energy = get_energy(x);
        numerical_gradient(x, grad);
        return energy;
    }


    virtual double add_energy_gradient_hessian(Array<double> x, Array<double> grad,
            Array<double> hess)
    {
        throw std::runtime_error("BasePotential::add_energy_gradient_hessian must be overloaded");
    }

    virtual double get_energy_gradient_hessian(Array<double> x, Array<double> grad,
            Array<double> hess)
    {
        double energy = get_energy_gradient(x, grad);
        numerical_hessian(x, hess);
        return energy;
    }

    virtual void numerical_gradient(Array<double> x, Array<double> grad, double eps=1e-6)
    {
        if (x.size() != grad.size()) {
            throw std::invalid_argument("grad.size() be the same as x.size()");
        }

        Array<double> xnew(x.copy());
        for (size_t i=0; i<xnew.size(); ++i){
            xnew[i] -= eps;
            double eminus = get_energy(xnew);
            xnew[i] += 2. * eps;
            double eplus = get_energy(xnew);
            grad[i] = (eplus - eminus) / (2. * eps);
            xnew[i] = x[i];
        }
    }

    virtual void get_hessian(Array<double> x, Array<double> hess)
    {
        Array<double> grad(x.size());
        get_energy_gradient_hessian(x, grad, hess);
    }

    virtual void numerical_hessian(Array<double> x, Array<double> hess, double eps=1e-6)
    {
        if (hess.size() != x.size()*x.size()) {
            throw std::invalid_argument("hess.size() be the same as x.size()*x.size()");
        }
        size_t const N = x.size();

        Array<double> gplus(x.size());
        Array<double> gminus(x.size());

        for (size_t i=0; i<x.size(); ++i){
            double xbackup = x[i];
            x[i] -= eps;
            get_energy_gradient(x, gminus);
            x[i] += 2. * eps;
            get_energy_gradient(x, gplus);
            x[i] = xbackup;

            for (size_t j=0; j<x.size(); ++j){
                hess[N*i + j] = (gplus[j] - gminus[j]) / (2.*eps);
            }
        }
        /*//print hessian
        std::cout<<""<<std::endl;
        std::cout<<""<<std::endl;
        for (size_t i=0; i<hess.size(); ++i){
            std::cout<<hess[i]<<" ";
            int j = i+1;
            if (j % (int) (3*sqrt(hess.size())) == 0)
                std::cout<<"\n"<<std::endl;
            else if (j % (int) sqrt(hess.size()) == 0)
                std::cout<<""<<std::endl;
            else if (j % 3 == 0){
                std::cout<<"  ";
            }
        }*/
    }

};
}

#endif