  We discuss localized ground states of the periodic Gross-Pitaevskii equation
in the framework of a quantum linear Schr\"odinger equation with effective
potential determined in self-consistent manner. We show that depending on the
interaction among the atoms being attractive or repulsive, bound states of the
linear self consistent problem are formed in the forbidden zones of the linear
spectrum below or above the energy bands. These eigenstates are shown to be
exact solitons of the GPE equation. The implications of this bound state
interpretation on the existence of a delocalization transition for
multidimensional solitons is briefly discussed.
