Benchmark full configuration interaction calculations on the helium dimer

Full configuration interaction calculations are presented for the helium dimer employing large basis sets. Using the best basis, which contains up to h-type basis functions and several closely spaced sets of bond functions, the interaction energy was calculated for a variety of internuclear distances in the range 4.0 to 12.0 bohr. The best calculated values for the He2 interaction energy are −10.947 K at 5.6 bohr (the van der Waals minimum) and +294.90 K at 4.0 bohr (on the repulsive wall). The interaction energy at 4.0 bohr differs significantly from the most recent semiempirical potential of Aziz and Slaman [J. Chem. Phys. 94, 8047 (1991)], indicating that this potential is too attractive around 4.0 bohr. Using a more generally accessible basis, containing only up to f-type basis functions and only one set of bond functions, the interaction energy was calculated to be −10.903 K at 5.6 bohr and +294.96 K at 4.0 bohr. These results show that functions of higher than f symmetry and bond functions distributed over several centers are necessary for obtaining highly accurate results, particularly at the van der Waals minimum. Our results may be used to benchmark more approximate methods. The CCSD(T) method is estimated to underestimate the full CI interaction energy by 0.33 K at 5.6 bohr and by 2.0 K at 4.0 bohr.