  In Ref 1[Phy. Rev. B 74, 195312(2006)] Fu and Kane propose a spin pump for
onedimensional (1D) insulating Hamiltonians. They claim that this spin pump is
a Z2 pump because For an isolated system, a single closed cycle of the pump
changes the expectation value of the spin at each end even when spin-orbit
interactions violate the conservation of spin. A second cycle, however, returns
the system to its original state. A Z2 topological invariant is proposed to
characterize the Z2 pump. In this comment we show their discussion on the spin
pump is inaccurate. Their reason why the isolated system return to its original
state after second cycle is unjustified and several claims contradict to this
return of the system are made in Ref 1. Detailed calculations and concrete
examples show the degeneracy of the first excited state at t = 0, T; is not
split by the electron-electron interaction in the way described in Ref 1 and
there is level crossing at t = T. In fact, despite of a detailed search, not a
single system behave as described in Fig. 1(d) in Ref 1 has been found. Thus we
conclude the isolated system won't return to its original state after two
cycles and the spin pump is not a Z2 pump in general.
