Combining multireference methods with the density matrix renormalization group

Standard multireference methods are limited by the size of the active orbital space.
Here, we present a combination of multireference configuration interaction (MR-CI) methods
with the density matrix renormalization group (DMRG). Therein, the MR-CI wavefunction is directly
approximated via the DMRG. This (1) allows for using much larger active spaces,
(2) alleviates the need for computing costly many-body density matrices,
and (3) does not require internal contraction, which decreases the variational flexibility.
Extensions of this approach to more accurate MR averaged quadratic coupled cluster,
MR linearized coupled cluster, and selected CI are straightforward.
We apply this method to the challenging chromium dimer.