A phenomenological momentum-independent (MID) model is constructed to describe the equation of state (EOS) for isospin asymmetric nuclear matter, especially the density dependence of the nuclear symmetry energy Esym(ρ). This model can reasonably describe the general properties of the EOS for symmetric nuclear matter and the symmetry energy predicted by both the sophisticated isospin and momentum dependent MDI model and the Skyrme-Hartree-Fock approach. We find that there exists a nicely linear correlation between Ksym and L as well as between J 0 /K 0 and K 0 , where L and Ksym represent, respectively, the slope and curvature parameters of the symmetry energy at the normal nuclear density ρ 0 , while K 0 and J 0 are, respectively, the incompressibility and the third-order derivative parameter of symmetric nuclear matter at ρ 0 . These correlations together with the empirical constraints on K 0 , L and Esym(ρ 0 ) lead to an estimation of −477 MeV K sat,2 −241 MeV for the second-order isospin asymmetry expansion coefficient for the incompressibility of asymmetric nuclear matter at the saturation point.
equation of state of nuclear matter, isospin, the symmetry energyThe study of the isospin degree of freedom in nuclear physics has recently attracted much attention due to the establishment of many radioactive beam facilities around the world. Besides the many existing radioactive beam facilities and their upgrades, such as the Cooling Storage Ring (CSR) facility at HIRFL in China [1] , many more are being constructed or under planning, including the Radioactive Ion Beam (RIB) Factory at RIKEN in Japan [2] , the FAIR/GSI in Germany [3] , SPIRAL2/GANIL in France [4] , and the Facility for Rare Isotope Beams (FRIB) in the USA [5] . These new facilities offer the possibility to study the properties of nuclear matter or nuclei under the extreme condition of large isospin asymmetry. The ultimate goal of such study is to extract information on the isospin dependence of in-medium nuclear effective interactions as well as the equation of state (EOS) of isospin asymmetric nuclear matter, particularly its isospin-dependent term or the density dependence of the nuclear symmetry energy. This knowledge, especially the latter, is important for understand-