Ab initio basis set and correlation limit interaction energies for He-He, He-H2, and H-H2

An ab initio search for the exact nonrelativistic interaction energies for He-He, He-H-2, and H-H-2 systems was made by estimating the basis set limit CCSD(T) interaction energies from the extrapolation of counterpoise (CP) interaction energies with aug-cc-pV5Z and aug-cc-pV6Z basis sets by 1/(X - 1)(3) (X = 5, 6) and correcting for the difference between the FCI and CCSD(T) energies. While the result for He-2 is in perfect agreement with the exact quantum Monte Carlo result, the result for H-3 suggests the barrier height for colinear H + H-2 --> H-2 + H reaction is close to 9.60 kcal/mol, similar to0.01 kcal/mol lower than the best result reported so far (C) 2001 Elsevier Science B.V. All rights reserved.