  We develop a generalized framework based on a Green's function formalism to
calculate the efficiency of multiexciton gen-eration in nanocrystal quantum
dots. The direct/indirect absorption and coherent/incoherent impact ionization
mechanisms, often used to describe multiexciton generation in nanocrystals, are
reviewed and rederived from the unified theory as certain approximations. In
addition, two new limits are described systematically - the weak Coulomb
coupling limit and the semi-wide band limit. We show that the description of
multiexciton generation in nanocrystals can be described as incoherent process
and we discuss the scaling of multiexciton generation with respect to the
photon energy and nanocrystal size. Illustrations are given for three prototype
systems: CdSe, InAs and silicon quantum dots.
