Why Nondynamic Correlation Matters for ππ Stacking? Lessons from the Benzene Dimer

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate benchmarking of ππ interactions is essential for understanding molecular behavior.
• Coupled cluster with singles, doubles, and perturbative triples [CCSD(T)] and fixed-node diffusion Monte Carlo (DMC) are leading computational methods.
• The parallel-displaced benzene dimer (BZPD) serves as a critical model system for evaluating these methods.

Purpose of the Study:

• To assess the performance of single-determinant diffusion Monte Carlo (SDDMC) in describing ππ interactions.
• To identify the limitations of SDDMC, particularly concerning weak nondynamic correlation effects.
• To compare SDDMC results with high-level coupled cluster calculations [CCSD(T)] for the BZPD system.

Main Methods:

• Utilized fixed-node diffusion Monte Carlo (DMC) with trial wave functions derived from Density Functional Theory (DFT) or Hartree-Fock.
• Employed symmetry-adapted perturbation theory and correlation energy decompositions to analyze interaction energies.
• Compared DMC results against benchmark coupled cluster with singles, doubles, and perturbative triples [CCSD(T)] calculations at the Complete Basis Set (CBS) limit.

Main Results:

• Single-determinant DMC (SDDMC) systematically underestimates interaction energies for the BZPD, indicating underbinding.
• The underbinding is attributed to the neglect of weak nondynamic correlation effects in the mean-field trial wave functions used in SDDMC.
• Reference calculations highlight the critical role of these subtle correlation effects in achieving accurate benchmark interaction energies.

Conclusions:

• Single-determinant DMC (SDDMC) has limitations in accurately describing ππ interactions due to insufficient treatment of nondynamic correlation.
• Achieving reliable benchmark quality for larger molecular complexes with DMC requires improved nodal descriptions.
• Further methodological development is needed to establish DMC as a robust benchmark method for noncovalent interactions.