  In describing the motion of atoms and clusters, we face with choosing quantum
mechanics or classical mechanics under different conditions. In principle,
there exist two criteria for this choice, but they do contradict in some cases
though they are in agreement for other cases. Actually, this problem is closely
related with the effective centre-of-mass method, the underlying application of
quantum mechanics. It is shown that quantum mechanics must be selected for
particle's motion when the de Broglie wave length of the mass centre is larger
than the particle size, and in such case the effective centre-of-mass can be
used in Quantum Mechanics. In order to test this conclusion, an
easy-manufactured experiment is suggested.
