double norm_dist_to(const Particle &p) {
        double x_diff = p.x - x;
        double y_diff = p.y - y;
        double z_diff = p.z - z;

        return max(MIN_DIST,
            sqrt(pow(x_diff, 2) + pow(y_diff, 2) + pow(z_diff, 2)));
    }