  A Bose-Einstein condensate (BEC) confined in a one-dimensional lattice under
the effect of an external homogeneous field is described by the
Gross-Pitaevskii equation. Here we prove that such an equation can be reduced,
in the semiclassical limit and in the case of a lattice with a finite number of
wells, to a finite-dimensional discrete nonlinear Schrodinger equation. Then,
by means of numerical experiments we show that the BEC's center of mass
exhibits an oscillating behavior with modulated amplitude; in particular, we
show that the oscillating period actually depends on the shape of the initial
wavefunction of the condensate as well as on the strength of the nonlinear
term. This fact opens a question concerning the validity of a method proposed
for the determination of the gravitational constant by means of the measurement
of the oscillating period.
