Binding energies and three body potentials of He3 and Ne3:: A hierarchical approach toward the basis set and correlation limit

The basis set limit electronic binding energies of small rare gas clusters X-n (X = He and Ne; n = 2 and 3) and nonadditive three body potential at the MP, CCSD, and CCSD(T) levels (coupled cluster single and double excitations with perturbative triples correction) were obtained assuming the correlation energies of the monomer and cluster have the same convergence behavior toward the corresponding basis set limits with correlation consistent aug-cc-pVXZ (X = 1)(2), T(3), Q(4), 5, 6) Basis set. The comparison of the estimated basis set limits with the estimates obtained from, 1/X-3 extrapolation schemes and the exact (reference) basis set limits shows that the new extrapolation scheme is capable of yielding a much more accurate estimate to the basis set limit than two-point 1/X-3 extrapoation scheme with small basis sets, though estimated basis set limits by both extrapolation schemes appear to converge as the basis set increases. The three body potentials of He-3 and Ne-3 are found negligible at their equilateral configurations near equilibrium. An effective procedure is explored to derive the basis set limit binding energies at the CCSD(T) level from the results at the MP2 level in a hierarchical manner based on the appropriate extrapolation of correlation consistent energies.