Accurately solving the electronic Schrödinger equation of atoms and molecules using explicitly correlated (r 12 -)MR-CI. VI. The helium dimer (He 2 ) revisited

A recent computation of the helium dimer interaction potential for intermediate distances (2 ≤ R/a0 ≤ 15) using the (explicitly correlated) r12-MR-ACPF (averaged coupled-pair functional) method [GDANITZ, R. J., 1999, Molec. Phys., 96, 1423] is extended to larger basis sets. Calculations including a large atom centred [11 s8p6d5f4g3h2ilk] set show that the convergence of the energy with respect to enlargement of the basis set has previously been considerably underestimated. The extension of the Pk potential (292.75 ± 0.01, −10.980 ± 0.004 and −4.620 = ± 0.002 K at R = 4, 5.6 and 7 a0) to distances between 2.5 and 15a0 (Pk∗)is close to a recent potential of van Mourik and Dunning [1999, J. Chem. Phys., 111, 9248] and it is believed to be the most accurate available. It gives rise to 4He2 molecular constants of (R) = 46.4 ± 1.4 Å and D0 = 1.67 ± 0.11 mK.