Driven similarity renormalization group: Third-order multireference perturbation theory

Abstract

A third-order multireference perturbation theory based on the driven similarity renormalization group approach (DSRG-MRPT3) is presented. The DSRG-MRPT3 method has several appealing features: a) it is intruder free, b) it is size consistent, c) it leads to a non-iterative algorithm with $\mathcal{O}(N^6)$ scaling, and d) it includes reference relaxation effects. The DSRG-MRPT3 scheme is benchmarked on the potential energy curves of F${}_2$, H${}_2$O${}_2$, C${}_2$H${}_6$, and N${}_2$ along the F-F, O-O, C-C, and N-N bond dissociation coordinates, respectively. The nonparallelism errors of DSRG-MRPT3 are consistent to those of CASPT3 and MRCISD, and show significant improvements over those obtained from DSRG second-order multireference perturbation theory. Our efficient implementation of the DSRG-MRPT3 based on factorized electron repulsion integrals enables studies of medium-sized open-shell organic compounds. This point is demonstrated with computations of the singlet-triplet splitting (${\Delta }_{\mathrm{ST}}=E_{\mathrm{T}}-E_{\mathrm{S}}$) of 9,10-anthracyne. At the DSRG-MRPT3 level of theory, our best estimate of the adiabatic ${\Delta }_{\mathrm{ST}}$ is 3.9 kcal mol${}^{-1}$, a value that is within 0.1 kcal mol${}^{-1}$ from multireference coupled cluster results.

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• A regularized second-order correlation method from Green's function theory
• Multireference Density Functional Theory for Describing Ground and Excited States with Renormalized Singles
• A benchmark study of core-excited states of organic molecules computed with the generalized active space driven similarity renormalization group
• Benchmarking CASPT3 Vertical Excitation Energies
• Connected three-body terms in single-reference unitary many-body theories: Iterative and perturbative approximations
• Renormalized Internally-Contracted Multireference Coupled Cluster with Perturbative Triples
• Complete Active Space Iterative Coupled Cluster Theory
• Equation-of-motion internally contracted multireference unitary coupled-cluster theory
• Improving the efficiency of the multireference driven similarity renormalization group via sequential transformation, density fitting, and the non-interacting virtual orbital approximation