Macroscopic quantum bound states of Bose Einstein condensates in optical lattices

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.