The rotational energy gain function for diatomic molecule, which is defined as the mean rotational energy gain per collision per molecule without pre-collision rotational energy, is examined by the Monte Carlo integration combined with the classical trajectory calculation using the diatomic Lennard-Jones potential, and then compared with the Parker s approximate theory. If is demonstrated that the calculated energy gain function is reasonable agreement with that of Parker s expression for the translational energy range E/ε_<LJ>≥10 and smaller than Parker s expression for E/ε_<LJ>≤10, and simulated rotational collision number is smaller than Parker s expression.
