/* @return the kinetic energy of the given four atoms. */
	private double getKE(int i, int j, int k, int l) {
		double ke = (atom[i].vx * atom[i].vx + atom[i].vy * atom[i].vy) * atom[i].mass
				+ (atom[j].vx * atom[j].vx + atom[j].vy * atom[j].vy) * atom[j].mass
				+ (atom[k].vx * atom[k].vx + atom[k].vy * atom[k].vy) * atom[k].mass
				+ (atom[l].vx * atom[k].vx + atom[l].vy * atom[l].vy) * atom[l].mass;
		return ke * EV_CONVERTER;
	}