  The density matrix renormalization group (DMRG) approach is extended to
complex-symmetric density matrices characteristic of many-body open quantum
systems. Within the continuum shell model, we investigate the interplay between
many-body configuration interaction and coupling to open channels. It is shown
that the DMRG technique applied to broad resonances in the unbound neutron-rich
nucleus 7He provides a highly accurate treatment of the coupling to the
non-resonant scattering continuum.
