Ab initio and DFT studies for accurate description of van der Waals interaction between He atoms

To evaluate the accuracy of density functional theory (DFT) for describing van der Waals interaction, we performed CCSD(T), MP4(SDTQ) and several DFT calculations for the He dimer. The results clarified that (1) CCSD(T) and MP4(SDTQ) with an `adjustedʹ diffuse basis-set reproduce the experimental binding energy as well as the equilibrium distance, (2) MP3 and MP2 underestimate the binding energy, (3) B3LYP and B3PW91 completely fail to describe the binding, (4) Perdew–Wang exchange-correlation functional and its modified ones can provide a He–He binding state, although the calculated binding energies are largely overestimated. The basis-set dependence of the binding energy was also discussed.