  We extend the active space decomposition method, recently developed by us, to
more than two active sites using the density matrix renormalization group
algorithm. The fragment wave functions are described by complete or restricted
active-space wave functions. Numerical results are shown on a benzene pentamer
and a perylene diimide trimer. It is found that the truncation errors in our
method decrease almost exponentially with respect to the number of
renormalization states M, allowing for numerically exact calculations (to a few
microhartrees or less) with M = 128 in both cases. This rapid convergence is
because the renormalization steps are used only for the interfragment electron
correlation.
