Multireference Driven Similarity Renormalization Group: A Second-Order Perturbative Analysis

We introduce a multireference version of the driven similarity renormalization group (DSRG) approach [ Evangelista , F. A. J. Chem. Phys. 2014 , 141 , 054109 ] based on a generalized reference wave function and operator normal ordering [ Kutzelnigg , W. ; Mukherjee , D. J. Chem. Phys. 1997 , 107 , 432 ]. We perform a perturbative analysis of the corresponding equations at second order and derive a novel multireference perturbation theory, termed DSRG-MRPT2. The DSRG-MRPT2 energy equation can be written in a simple and compact form and can be solved via a noniterative procedure that requires at most the three-body density cumulant of the reference. Importantly, even at the perturbation level, the multireference DSRG is free from the intruder-state problem. We propose an optimal range of the DSRG flow parameter that consistently yields reliable potential energy curves with minimal nonparallelism error. We find that the DSRG-MRPT2 can describe the potential energy curves of HF and N2, and the singlet-triplet gap of p-benzyne with an accuracy similar to that of other multireference perturbation theories.