double form_volume(double radius);

double Iq(double q,
          double fractal_dim_mass,
          double fractal_dim_surf,
          double rg_cluster,
          double rg_primary);

static double _mass_surface_fractal_kernel(double q,
          double fractal_dim_mass,
          double fractal_dim_surf,
          double rg_cluster,
          double rg_primary)
{
     //computation
    double tot_dim = 6.0 - fractal_dim_surf - fractal_dim_mass;
    fractal_dim_mass /= 2.0;
    tot_dim /= 2.0;

    double rc_norm = rg_cluster * rg_cluster / (3.0 * fractal_dim_mass);
    double rp_norm = rg_primary * rg_primary / (3.0 * tot_dim);

    //x for P
    double x_val1 = 1.0 +  q * q * rc_norm;
    double x_val2 = 1.0 +  q * q * rp_norm;

    double inv_form = pow(x_val1, fractal_dim_mass) * pow(x_val2, tot_dim);

    //another singular
    if (inv_form == 0.0) return 0.0;

    double form_factor = 1.0;
    form_factor /= inv_form;

    return (form_factor);
}
double form_volume(double radius){

    return M_4PI_3*radius*radius*radius;
}

double Iq(double q,
          double fractal_dim_mass,
          double fractal_dim_surf,
          double rg_cluster,
          double rg_primary)
{
    return _mass_surface_fractal_kernel(q,
            fractal_dim_mass,
            fractal_dim_surf,
            rg_cluster,
            rg_primary);
}