public Complex getDelta(int index, double energy) {
        MatterInterval inl = intervals.get(index);
        if (index == 0) {
            // Fictitious interval
            return inl.getWaveNumber(energy).times(-inl.getWidth());
        } else if (index == 1) {
            return new Complex(0.0, 0.0);
        } else {
            // FIXME Check sum of the intervals widths from 0 to
            // index.
            // The intervals must not have equal widths, for now the
            // opposite is
            // assumed -- TEST THE LINE BELOW
            return inl.getWaveNumber(energy).times(getSubStackWidth(1, index));
        }
    }