Accurately solving the electronic Schrodinger equation of atoms and molecules using explicitly correlated (r12-)MR-CI IV The helium dimer (He2)

The helium dimer interaction potential is computed using the recently proposed (explicitly correlated) r(12)-MR-ACPF (averaged coupled-pair functional) method and a [11s8p6d5f4g] basis set. With an MR-ACPF ansatz that contains 121 references we obtain interaction energies that are close to full CI. In a smaller reference space containing 9 functions, however, even by successively adding [3h] and [2i] functions to the basis set mentioned above, the basis set limit could not be reached. While convergence to the basis set limit is slow, it nevertheless is monotonic and therefore allows for extrapolation to the limit. We obtain basis set corrections at R = 4a(0) and 5.6a(0) which we further extend to all distances and which we apply to the potential energy curve mentioned above. From our calculations, we conclude that a very recent potential which has been calculated using the SAPT (symmetry adapted perturbation theory) method, and which previously was assumed to be the most accurate available, is insufficiently repulsive at short distances. We correct our extrapolated potential for retardation and finally calculate the expectation value of the interatomic distance ([R]) and dissociation energy (D-0) by solving the Schrodinger equation of the vibrating He-4(2). Our results ([R] = 41 +/- 13 Angstrom and D-0 = 2.2 +/- 1.0 mK) are in acceptable agreement with very recent calculations in the literature, but they disagree with a recent experiment.