public double fx_i() {
		double d = 0.0;
		if (hasCharge_i && hasCharge_j)
			d += chargeChargeVsX_i();
		if (hasDipole_i && hasDipole_j)
			d += dipoleDipoleVsX_i();
		if (hasCharge_i && hasDipole_j)
			d += pjqiVsX_i();
		if (hasDipole_i && hasCharge_j)
			d += piqjVsX_i();
		return d * rCD;
	}