  It is well-known that not only the orbital ordering but also the choice of
the orbitals themselves as the basis may significantly influence the
computational efficiency of density-matrix renormalization group (DMRG)
calculations. In this study, for assessing the efficiency of using various
natural orbitals (NOs) as the DMRG basis, we performed benchmark DMRG
calculations with different bases, which included the NOs obtained by various
traditional electron correlation methods, as well as NOs acquired from
preliminary moderate DMRG calculations (e.g., preserved states less than 500).
The tested systems included N$_2$, transition metal Cr$_2$ systems, as well as
1-D hydrogen polyradical chain systems under equilibrium and dissociation
conditions and 2-D hydrogen aggregates. The results indicate that a good
compromise between the requirement for low computational costs of acquiring NOs
and the demand for high efficiency of NOs as the basis of DMRG calculations may
be very dependent on the studied systems' diverse electron correlation
characteristics and the size of the active space. It is also shown that a
DMRG-complete active space configuration interaction (DMRG-CASCI) calculation
in a basis of carefully chosen NOs can provide a less expensive alternative to
the standard DMRG-CASSCF calculation and avoid the convergence difficulties of
orbital optimization for large active spaces. The effect of different NO
ordering schemes on DMRG-CASCI calculations is also discussed.
