The convergence of the barrier height for the O+H2rarr;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 2prarr;2prsquo;correlation terms to the active space lowers the barrier to the O+H2reaction by about 2.0 kcal/mol, but addition of 3dand other terms has little additional effect. Multireference singles and doubles contracted CI plus Davidsonrsquo;s correction calculations using a lsqb;5s5p3d2f1g/4s3p2d1frsqb; basis set with a 5sgr;2pgr; 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.
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