Convergence to the configuration-set limit in multireference configuration-interaction calculations on the He dimer

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Abstract
The multireference configuration-interaction (MR-CI) method is used to calculate the binding energy of the He dimer. The convergence of the binding energy to the configuration-set limit (full-CI) is followed by progressively extending the multireference configuration set. Two variants of the Pople size-extensivity correction are applied. The distance dependence of the corrections and hence the effect upon the binding energy turns out to be very small. The effect of orbital optimization is studied and it is shown that it is sufficient to optimize the orbitals used for the multireference space in an atomic multiconfiguration self-consistent field (MCSCF) calculation. In a basis of 50 atomic orbitals, the full-CI binding energy of −9.08 K can be reproduced to 0.00 K (0.02 K) in calculations using only 37 (27) reference configurations, built from the atomic 1s, 2s, 2p, and 3s natural orbitals. Using a very large basis, the 37-reference set gives a best binding energy of −10.87 K, in satisfactory agreement with Aziz’s recent semiempirical result of −10.95 K. These findings suggest that the MR-CI method can be developed into an efficient tool for calculating accurate van der Waals interaction energies for larger systems.