We present a molecular dynamic simulation (MD) to study relaxations in a Lennard-Jones liquid with an additional molecular friction term. To avoid crystallization the atomic size is varied. The simulation is done for-a fixed density. The problems of MD-simulations in undercooled systems with a Newtonian friction term and under conservation of the total momentum are overcome by our analysis. The origin of friction terms in a more microscopical approach is discussed. We calculate trajectories connecting the minima in the phase space of the total system. It is shown that both structural relaxations and vibrations of atoms around their fixed positions can be analyzed by those so-called pseudotrajectories used here. The size of cooperatively rearranging regions and the length of jumps between neighbored structures are studied.