Electron correlation in benzene: A QMC study of neutral and charged states

In this paper, we employ variational Monte Carlo and fixed-node diffusion Monte Carlo (FN-DMC) methods to investigate electron correlation effects in neutral and charged benzene molecules. Using orbitals from Hartree-Fock (HF) and density functional theory calculations, FN-DMC with the frozen-core approximation yields correlation energies consistent with CCSD(T) and ph-AFQMC, while all-electron FN-DMC lowers the correlation energy by ∼0.27 a.u., highlighting the role of core-electron contributions. On average, all-electron FN-DMC predicts vertical [VIP: 0.3477(34) a.u.] and adiabatic [AIP: 0.3433(39) a.u.] ionization potentials, both slightly above the experimental value [0.339 70(2) a.u.]. Zero-point vibrational energy (ZPVE) corrections adjust the VIP and AIP by -0.012 and +0.0037 a.u., respectively, improving agreement with experiment. Compared to HF, FN-DMC incorporates an additional 0.055 a.u. (≈1.5 eV) of correlation energy into the IP, demonstrating its effectiveness in capturing electron correlation in aromatic systems.