  The calculation of the third order susceptibility still is a long standing
fundamental problem of particular importance in nonlinear nanooptics: Indeed,
cancellation of size-dependent terms coming from uncorrelated excitations is
expected, but up to now shown for very simple Hamiltonians only. Using a
many-body theory recently developed to handle interacting close-to-bosons, we
prove it here for \emph{arbitrary} H. This new formalism actually provides the
first clean way to calculate nonlinear susceptibilities, with results different
from previous ones.
