The Center-of-Mass of A Few-Body Quantum System In An Effective Central Field

The center-of-mass(CM) of a few-body quantum system with a central field is discussed. If the particles are in the designative eigenstates, the CM coordinates of the system can be well-defined. In the CM bag model as well as in other models with central fields, the CM-freedom separation rule and effective nucleon electro-magnetic currents can be presented without any undetermined parameter.