  Cosmological models with different types of Dark Energy are becoming viable
alternatives for standard models with the cosmological constant. Yet, such
models are more difficult to analyze and to simulate. We present analytical
approximations and discuss ways of making simulations for two families of
models, which cover a wide range of possibilities and include models with both
slow and fast changing ratio w=p\rho. More specifically, we give analytical
expressions for the evolution of the matter density parameter Omega_m(z) and
the virial density contrast Delta_c at any redshift z. The latter is used to
identify halos and to find their virial masses. We also provide an
approximation for the linear growth factor of linear fluctuations between
redshift z=40 and z=0. This is needed to set the normalization of the spectrum
of fluctuations. Finally, we discuss the expected behavior of the halo mass
function and its time evolution.
