  We use CheMPS2, our free open-source spin-adapted implementation of the
density matrix renormalization group (DMRG) [Wouters et al., Comput. Phys.
Commun. 185, 1501 (2014)], to study the lowest singlet, triplet, and quintet
states of the oxo-Mn(Salen) complex. We describe how an initial approximate
DMRG calculation in a large active space around the Fermi level can be used to
obtain a good set of starting orbitals for subsequent complete-active-space or
DMRG self-consistent field (CASSCF or DMRG-SCF) calculations. This procedure
mitigates the need for a localization procedure, followed by a manual selection
of the active space. Per multiplicity, the same active space of 28 electrons in
22 orbitals (28e, 22o) is obtained with the 6-31G*, cc-pVDZ, and ANO-RCC-VDZP
basis sets (the latter with DKH2 scalar relativistic corrections). Our
calculations provide new insight into the electronic structure of the quintet.
