  We report a spin-free formulation of the multireference (MR) driven
similarity renormalization group (DSRG) by employing the ensemble normal
ordering of Mukherjee and Kutzelnigg [W. Kutzelnigg and D. Mukherjee, J. Chem.
Phys. 107, 432 (1997)]. This ensemble averages over all microstates for a given
total spin quantum number and, therefore, it is invariant with respect to SU(2)
transformations. As such, all equations may be reformulated in terms of
spin-free quantities and they closely resemble those of spin-adapted
closed-shell coupled cluster (CC) theory. The current implementation is used to
assess the accuracy of various truncated MR-DSRG methods (perturbation theory
up to third order and iterative methods with single and double excitations) in
computing the constants of thirty-three first-row diatomic molecules. The
accuracy trends for these first-row diatomics are consistent with our previous
benchmark on a small subset of closed-shell diatomic molecules. We then present
the first MR-DSRG application on transition-metal complexes by computing the
spin splittings of the [Fe(H$_2$O)$_6$]$^{2+}$ and [Fe(NH$_3$)$_6$]$^{2+}$
molecules. Focal point analysis (FPA) shows that third-order perturbative
corrections are essential to achieve reasonably converged energetics. A FPA
based on the linearized MR-DSRG theory with one- and two-body operators and up
to a quintuple-$\zeta$ basis set predicts the spin splittings of
[Fe(H$_2$O)$_6$]$^{2+}$ and [Fe(NH$_3$)$_6$]$^{2+}$ to be $-35.7$ and $-17.1$
kcal mol$^{-1}$, respectively, showing good agreement with results of local CC
theory with singles, doubles, and perturbative triples.
