A new ab initio potential energy curve for the helium dimer

The He-2 interaction potential has been computed employing augmented correl ation-consistent basis sets and several methods beyond those commonly used to recover electron correlation, namely fifth-order Moller-Plesset perturba tion theory (MP5), coupled cluster theory with full triple excitations (CCS DT), and full configuration interaction (FCI). Calculations employing the l argest basis sets were not feasible for the most sophisticated methods (MP5 , CCSDT, and FCI). Nonetheless, because of the rapid convergence of the cal culated interaction energy differences with basis set, it was possible to o btain reliable estimates of the complete basis set MP5, CCSDT, and FCI resu lts. The MP5 method is predicted to yield a well depth of 10.67 +/- 0.03 K at the complete basis set (CBS) limit. Thus, the accuracy of the MP5 method is comparable to that of the CCSD(T) method which yields a D-e of 10.68 +/ - 0.02 K. The CCSDT method yields an estimated well depth of 10.98 +/- 0.03 K. Thus, the full effect of connected triple excitations on the He-2 well depth is 1.74 K (CCSDT-CCSD). Comparing the basis set dependence of the CCS DT and FCI well depths, the effect of connected quadruple excitations on th e well depth is estimated to be just 0.015-0.020 K. Thus, the current calcu lations predict the FCI He-2 well depth to be 11.00 +/- 0.03 K, in excellen t agreement with the recent quantum Monte Carlo calculations of Anderson [J . Chem. Phys. 99, 345 (1993)]. Taking advantage of the rapid convergence of the full triples and quadruples correlation corrections to the CCSD(T) int eraction energy with basis set, an estimated FCI/CBS potential energy curve has been constructed for He-2 from 3.5 to 15.0 a(0). The resulting curve l ies between the HFD-B3-FCI1 and SAPT2 semi-empirical potentials of Aziz , b eing closer to the SAPT2 potential. (C) 1999 American Institute of Physics. [S0021-9606(99)30144-6].