Role of Exchange in Density Functional Theory for Weakly-Interacting Systems: Quantum Monte Carlo Analysis of Electron Density and Interaction Energy

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We analyze the density functional theory (DFT) description of weak interactions by employing diffusion and reptation quantum Monte Carlo (QMC) calculations, for a set of benzene-molecule complexes. While the binding energies depend significantly on the exchange correlation approximation employed for DFT calculations, QMC calculations show that the electron density is accurately described within DFT, including the quantitative features in the reduced density gradient. We elucidate how the enhancement of the exchange energy density at a large reduced density gradient plays a critical role in obtaining accurate DFT description of weakly-interacting systems.