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This study examines several techniques to improve the efficiency of thelinearized multireference driven similarity renormalization group truncated toone- 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 intothe Hamiltonian. This new approach is combined with density fitting (DF) toreduce the storage cost of two-electron integrals. To further avoid the storageof large four-index intermediates, we propose a non-interacting virtual orbit(NIVO) approximation in which tensor blocks labeled by three and four virtualindices are neglected. The NIVO approximation reduces the computational costprefactor of the MR-LDSRG(2) bringing it closer to that of coupled cluster withsingles and doubles (CCSD). We test the effect of the DF and NIVOapproximations on the MR-LDSRG(2) and sq-MR-LDSRG(2) methods by computingproperties of eight diatomic molecules. The diatomic constants obtained byDF-sq-MR-LDSRG(2)+NIVO are found to be as accurate as those from the originalMR-LDSRG(2) and coupled cluster theory with singles, doubles, and perturbativetriples. Finally, we demonstrate that the DF-sq-MR-LDSRG(2)+NIVO scheme can beapplied to chemical systems with more than 550 basis functions by computing theautomerization energy of cyclobutadiene with a quintuple-zeta basis set. Thepredicted automerization energy is found similar to the values computed withMukherjee's state-specific multireference coupled cluster theory with singlesand doubles.

Improving the efficiency of the multireference driven similarity  renormalization group via sequential transformation, density fitting, and the  non-interacting virtual orbital approximation