  The background field method is adopted for studying the dynamics of coherent
states within an interacting scalar field theory. Focusing on a coherent state
that corresponds to the homogeneous condensate, the quantum depletion of the
expectation value of the field-operator is demonstrated to be due to the
annihilation of the condensate constituents into relativistic quanta. Moreover,
due to the fact that the initial field acceleration and energy for the
non-squeezed coherent states are determined in terms of bare coupling constant,
instead of the renormalized one, the appearance of perturbative singularities
is shown to be inevitable. In other words, consistency of these states requires
the finiteness of the bare coupling constant, through the resummation.
