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

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The driven similarity renormalization group (DSRG) is a powerful method for electronic structure calculations.
• Existing DSRG methods face challenges in describing systems with multiple important electronic configurations (multireference systems).
• The intruder-state problem is a common artifact in multireference perturbation theories.

Purpose of the Study:

• To introduce a multireference extension of the DSRG approach.
• To develop a novel second-order multireference perturbation theory based on the DSRG framework, termed DSRG-MRPT2.
• To address the limitations of existing multireference methods, including the intruder-state problem.

Main Methods:

• Development of a multireference DSRG approach using a generalized reference wave function and operator normal ordering.
• Perturbative analysis of the DSRG equations at second order to derive the DSRG-MRPT2 energy expression.
• Identification of an optimal range for the DSRG flow parameter to ensure accurate potential energy curves.

Main Results:

• The DSRG-MRPT2 energy equation is compact and solvable noniteratively using at most three-body density cumulants.
• The DSRG-MRPT2 method is inherently free from the intruder-state problem, even at the perturbation level.
• DSRG-MRPT2 accurately describes potential energy curves for HF and N2, and the singlet-triplet gap of p-benzyne, comparable to other state-of-the-art multireference perturbation theories.

Conclusions:

• The developed DSRG-MRPT2 method offers a robust and accurate approach for multireference electronic structure problems.
• The method provides reliable potential energy curves with minimal nonparallelism error.
• DSRG-MRPT2 represents a significant advancement in computational quantum chemistry for challenging molecular systems.