PairwiseKernel.h

#pragma once 

#include <cmath>
#include "json/json.h"

namespace SSAGES
{

    class PairwiseKernel
    {
    public:


        virtual double Evaluate(double rij, double& df) const = 0;


        static PairwiseKernel* Build(const Json::Value& json, const std::string& path);
    };


    class GaussianPK : public PairwiseKernel
    {
    private: 
        double mu_; 
        double sigma_; 

    public:

        GaussianPK(double mu, double sigma) : 
        mu_(mu), sigma_(sigma) {}


        double Evaluate(double rij, double& df) const
        {
            const auto dx = (rij - mu_)/sigma_;
            const auto f = exp( - dx*dx/2.);
            const auto pre = - dx/sigma_;
            
            df = pre * f;
            return  f;
        }


        static GaussianPK* Build(const Json::Value& json, const std::string& path);
    };


    class RationalSwitchPK : public PairwiseKernel
    {
    private: 
        double d0_; 
        double r0_; 
        int n_, m_; 
    
    public:

        RationalSwitchPK(double d0, double r0, int n, int m) : 
        d0_(d0), r0_(r0), n_(n), m_(m) {}


        double Evaluate(double rij, double& df) const override
        {
            const auto xarg = (rij - d0_)/r0_;
            const auto xn = std::pow(xarg, n_);
            const auto xm = std::pow(xarg, m_);
            const auto f = (1.-xn)/(1.-xm);
            
            df = f/(d0_-rij)*(n_*xn/(1.-xn)+m_*xm/(xm-1.));
            return  f;
        }


        static RationalSwitchPK* Build(const Json::Value& json, const std::string& path);
    };
}