private double multiplySigmaFor(int idOfI, int idOfJ) {

		/* same element */
		if (idOfI == idOfJ)
			return getElement(idOfI).getSigma() * getElement(idOfJ).getSigma();

		/* others */
		if (affinity.isLBMixed(getElement(idOfI), getElement(idOfJ)))
			return getElement(idOfI).getSigma() * getElement(idOfJ).getSigma();

		double x = affinity.getSigma(getElement(idOfI), getElement(idOfJ));
		return x * x;

	}