  The elliptic flow for $Z\le2$ particles in heavy ion collisions at energies
from several tens to several hundreds MeV per nucleon is investigated by means
of transport model,i.e. a new version of the Improved Quantum Molecular
Dynamics model (ImQMD05). In this model, a complete Skyrme potential energy
density functional is employed. The influence of different effective
interactions and medium corrections of nucleon-nucleon cross sections on the
elliptic flow are studied. Our results show that a soft nuclear equation of
state and incident energy dependent in-medium nucleon-nucleon cross sections
are required for describing the excitation function of the elliptic flow at
intermediate energies. The size dependence of transition energies for the
elliptic flow at intermediate energies is also studied. The system size
dependence of transition energies fits a power of system size with a exponent
of 0.223.
