We study the accuracy of the time-dependent self-consistent-field approximation for collinear inelastic collisions between an atom and a diatomic molecule. Individual state-to-state transition probabilities, total energy transfer. and the global description of the wavefunction is considered. It is shown that errors in these properties may be very considerable and they depend strongly on the initial form of the wave packet. For the individual state-to-state transition probabilities we show, however, that the errors can be minimized dramatically by a proper choice of the initial average momentum of the wave packet.