Why Nondynamic Correlation Matters for <i>ππ</i> Stacking? Lessons from the Benzene Dimer

Two leading methods for benchmarking <i>ππ</i> interactions are fixed-node diffusion Monte Carlo (DMC) and coupled cluster with singles, doubles, and perturbative triples [CCSD(T)]. The parallel-displaced benzene dimer (BZPD) is a key model for assessing the performance of theoretical approaches in describing these interactions. Reference calculations, symmetry-adapted perturbation theory, and correlation energy decompositions highlight the subtle but critical role of weak nondynamic correlation effects in achieving benchmark interaction energies for BZPD. While single-determinant DMC (SDDMC), using nodes from DFT or Hartree-Fock, performs well for many noncovalent systems, our analysis shows that neglecting weak nondynamic correlations in mean-field trial wave functions leads to systematic underbinding in SDDMC energy differences, in contrast to CCSD(T)/CBS results. These findings underscore the limitations of SDDMC for describing <i>ππ</i> interactions and emphasize the need for improved nodal descriptions to establish DMC as a reliable benchmark method for larger complexes.