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–H2, and H–H2 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 He2 is in perfect agreement with the exact quantum Monte Carlo result, the result for H3 suggests the barrier height for colinear H+H2→H2+H reaction is close to 9.60 kcal/mol, ∼0.01 kcal/mol lower than the best result reported so far