the NGCG fluid into two components, one is the dark energy component, and the other is the dark matter component, $$\rho_{\rm Ch}=\rho_X+\rho_{dm}~.$$ Note that the pressure of the NGCG fluid is provided only by the dark energy component, namely $p_{\rm Ch}=p_X$. Therefore, the energy density of the dark energy ingredient can be given $$\rho_X={p_{\rm Ch}\over w_X}={A a^{-3(1+w_X)(1+\alpha)}\over
[Aa^{-3(1+w_X)(1+\alpha)}+Ba^{-3(1+\alpha)}]^{\alpha\over
1+\alpha}}~~,\label{rhox}$$ and then the energy density of the dark matter component can also be obtained $$\rho_{dm}={Ba^{-3(1+\alpha)}\over
[Aa^{-3(1+w_X)(1+\alpha)}+Ba^{-3(1+\alpha)}]^{\alpha\over
1+\alpha}}~~.\label{rhodm}$$ From these expressions one obtains the scaling behavior of the energy densities $${\rho_{dm}\over\rho_X}={B\over
A}a^{3w_X(1+\alpha)}~~.\label{scaling}$$ We see explicitly from it that there must exist an energy flow between dark matter and dark energy provided that $\alpha\neq 0$. When $\alpha>0$, the transfer direction of the energy flow is from dark matter to dark energy; when $\alpha<0$, just the reverse. Therefore, it is clear that the parameter $\alpha$ characterizes the interaction between dark energy and dark matter. This is the reason for that we call $\alpha$ the interaction parameter.

The parameters $A$ and $B$ can be expressed in terms of current cosmological observables. From Eq.(\[chrho\]), it is easy to get $$A+B=\rho_{\rm Ch0}^\eta~,$$ where $\eta=1+\alpha$ is used to characterize the interaction for simplicity, thus we have $$A=\rho_{\rm Ch0}^\eta 