An accurate calculation of the first-order interaction energy for the helium dimer

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Abstract
The first-order interaction energy between two He atoms is calculated for a range of interatomic distances with a large explicitly correlated basis set reproducing over 99.998% of the correlation energy of the He atom. The interaction energy obtained in basis sets of increasing size converges to within 0.1 μhartree. This accuracy is comparable to accuracy of calculations which use orbital basis sets of spdfgh quality. Our results agree well with values extracted from experiments. We also show that our largest He wave functions behave properly in the outer region of the He atom.