Benchmark calculations with correlated molecular wavefunctions.  XIII. Potential energy curves for He₂, Ne₂, and Ar₂ using correlation consistent basis sets through augmented sextuple zeta

The potential energy curves of the rare gas dimers He-2, Ne-2, and Ar-2 have been computed using correlation consistent basis sets ranging from singly augmented aug-cc-pVDZ sets through triply augmented t-aug-cc-pV6Z sets, with the augmented sextuple basis sets being reported herein. Several methods for including electron correlation were investigated, namely Moller-Plesset perturbation theory (MP2, MP3 and MP4) and coupled cluster theory [CCSD and CCSD(T)]. For He-2 CCSD(T)/d-aug-cc-pV6Z calculations yield a well depth of 7.35 cm(-1) (10.58 K), with an estimated complete basis set (CBS) limit of 7.40 cm(-1) (10.65 K). The latter is smaller than the 'exact' well depth (Aziz, R. A., Janzen, A. R., and Moldover, M. R., 1995, Phys. Rev. Lett., 74, 1586) by about 0.2 cm(-1) (0.35 K). The Net well depth, computed with the CCSD(T)/d-aug-cc-pV6Z method, is 28.31 cm(-1) and the estimated CBS limit is 28.4 cm(-1) approximately 1 cm(-1) smaller than the empirical potential of Aziz, R. A., and Slaman, M., J., 1989, Chem. Phys., 130, 187. Inclusion of core and core-valence correlation effects has a negligible effect on the Nea well depth, decreasing it by only 0.04 cm(-1). For Ar-2, CCSD(T)/ d-aug-cc-pV6Z calculations yield a well depth of 96.2 cm(-1). The corresponding HFDID potential of Aziz, R. A., 1993, J. chem. Phys., 99, 4518 predicts of D-e of 99.7 cm(-1). Inclusion of core and core-valence effects in Ar-2 increases the well depth and decreases the discrepancy by approximately 1 cm(-1).