  The first nonperturbative version of the multireference driven similarity
renormalization group (MR-DSRG) theory [C. Li and F. A. Evangelista, J. Chem.
Theory Comput. $\mathbf{11}$, 2097 (2015)] is introduced. The renormalization
group structure of the MR-DSRG equations ensures numerical robustness and
avoidance of the intruder state problem, while the connected nature of the
amplitude and energy equations guarantees size consistency and extensivity. We
approximate the MR-DSRG equations by keeping only one- and two-body operators
and using a linearized recursive commutator approximation of the
Baker--Campbell--Hausdorff expansion [T. Yanai and G. K.-L. Chan, J. Chem.
Phys. $\mathbf{124}$, 194106 (2006)]. The resulting MR-LDSRG(2) equations
contain only 39 terms and scales as ${\cal O}(N^2 N_{\rm P}^2 N_{\rm H}^2)$
where $N_{\rm H}$, $N_{\rm P}$, and $N$ correspond to the number of hole,
particle, and total orbitals, respectively. Benchmark MR-LDSRG(2) computations
on the hydrogen fluoride and molecular nitrogen binding curves and the
singlet-triplet splitting of $p$-benzyne yield results comparable in accuracy
to those from multireference configuration interaction, Mukherjee
multireference coupled cluster theory, and internally-contracted multireference
coupled cluster theory.
