  Symmetry energy for asymmetric nuclear matter at subsaturation densitieswas
investigated in the framework of an isospin-dependent quantum molecular
dynamics model.Asingle ratio of neutrons and protons is comparedwith the
experimental data of Famiano et al. [Phys. Rev. Lett. 97, 052701 (2006)]. We
have also performed a comparison for the double ratio with experimental as well
as different theoretical results of Boltzmann-Uehling-Uhlenbeck in 1997,
Isospin-dependent Boltzmann-Uehling-Uhlenbeck in 2004,
Boltzmann-Nordeim-Vlasov, and Improved QuantumMolecular Dynamics models. It is
found that the double ratio predicts the softness of symmetry energy, which is
a little underestimated in the single ratio. Furthermore, the study of the
single ratio is extended for different kinds of fragments, while the double
ratio is extended for different neutron-rich isotopes of Sn.
