  This study examines several techniques to improve the efficiency of the
linearized multireference driven similarity renormalization group truncated to
one- and two-body operators [MR-LDSRG(2)]. We propose a sequential MR-LDSRG(2)
[sq-MR-LDSRG(2)] approach, in which one-body rotations are folded exactly into
the Hamiltonian. This new approach is combined with density fitting (DF) to
reduce the storage cost of two-electron integrals. To further avoid the storage
of large four-index intermediates, we propose a non-interacting virtual orbit
(NIVO) approximation in which tensor blocks labeled by three and four virtual
indices are neglected. The NIVO approximation reduces the computational cost
prefactor of the MR-LDSRG(2) bringing it closer to that of coupled cluster with
singles and doubles (CCSD). We test the effect of the DF and NIVO
approximations on the MR-LDSRG(2) and sq-MR-LDSRG(2) methods by computing
properties of eight diatomic molecules. The diatomic constants obtained by
DF-sq-MR-LDSRG(2)+NIVO are found to be as accurate as those from the original
MR-LDSRG(2) and coupled cluster theory with singles, doubles, and perturbative
triples. Finally, we demonstrate that the DF-sq-MR-LDSRG(2)+NIVO scheme can be
applied to chemical systems with more than 550 basis functions by computing the
automerization energy of cyclobutadiene with a quintuple-zeta basis set. The
predicted automerization energy is found similar to the values computed with
Mukherjee's state-specific multireference coupled cluster theory with singles
and doubles.
