Extended active space CASSCF/MRSD CI calculations of the barrier height for the reaction O+H2→OH+H

The convergence of the barrier height for the O+H2→OH+H reaction has been studied as a function of the size of the active space and basis set completeness. The barrier height is rapidly convergent with respect to expansion of the active space. Addition of 2p→2p′ correlation terms to the active space lowers the barrier to the O+H2 reaction by about 2.0 kcal/mol, but addition of 3d and other terms has little additional effect. Multireference singles and doubles contracted CI plus Davidson’s correction calculations using a [5s5p3d2f1g/4s3p2d1f] basis set with a 5σ2π active space lead to a barrier height of 12.7 kcal/mol. Including an estimate of the CI contraction error and basis set superposition error leads to 12.4 kcal/mol as the best estimate of the barrier height.