Pii: 0375-9601(78)90317-1

We havepreviously shown [1] that if theposition operator is definedas in ref. [21, themovementof the mean position ofa freeparticleobeys the classical equation u = P/P 0 where!’0 is the total energy, including the restmass. Conversely, it will be demonstrated here that the validity of this equation implies that, for spinless particles, the position operator is thatof ref. [2]. For spin 1/2 particles, however, another~choice is also possible (eq. (7)). The correspohding value of the orbital angular momentum in the latter case is unity, whereas for the state of ref. [2] it is zero.