On the other hand, the energy gap histogram for S2→S1 hoppings resembles an exponential distribution (Figure -bottom). For 6-AIH + , the mean value and standard deviations are 0.23 and 0.20 eV while for 7-AIH + they are 0.17 and 0.14 eV. Such a gap distribution reflect the small energy gap between the S2/S1 intersection and the S2 minimum.