The nonperturbative version of the multireference driven similarity renormalization group [MR-LDSRG(2)] is a robust quantum chemistry method that is free from the intruder-state problem. However, our previous implementation is limited to about 250 orbitals due to the storage of four-index tensors. In this work, we propose improved methods that overcome this limitation by combining the following techniques. Firstly, electron repulsion integrals are factorized using density fitting. Secondly, we introduce the “non-interacting virtual orbital” approximation to alleviate the storage requirement of four-index intermediate tensors by ignoring all commutator contributions labeled by three or four virtual indices. Meanwhile, we investigate the sequential transformation approach to evaluating the MR-LDSRG(2) transformed Hamiltonian, which consists of two steps: (1) the contribution of single substitution amplitudes are first folded exactly in via an efficient orbital rotation into the density-fitted integrals, and (2) double substitutions are evaluated using the Baker-Campbell-Hausdorff formula. The sequential transformation approach provides advantageously simple formulas that simplify future developments on MR-DSRG methods. Finally, we show that the combinations of reported approaches are capable of treating chemical systems with more than 500 virtual orbitals as accurately as the original MR-LDSRG(2) theory.

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

2019 Spring National Meeting

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