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We perform the fixed-node diffuse Monte Carlo (FN DMC) calculations todetermine the barrier height and reaction energy of a critical reaction, theH-transfer reaction from syn-CH3CHOO to vinyl hydroperoxide. The FN DMC barrierheight is found to be 16.60+/-0.35 kcal/mol which agrees well with theexperimental measurement within a few tenths of kcal, justifying thereliability of the FN DMC method for predicting barrier height of the rapidunimolecular reaction of Criegee intermediates. By comparing the predictionsfrom the CCSD(t), G3 (MCG3), DFT and MP2 methods with respect to the FN DMCresults and available experiment measurement, we found that the CCSD(t) barrierheights agree with the FN DMC counterpart within statistical errors, and iswithin a closer agreement with experiment and FN DMC prediction than theG3(MCG3) models. Barrier heights predicted from the relatively more economicDFT methods are within a few tenths kcal of the FN DMC prediction. MP2 methodseverely underestimates the barrier height. FN DMC prediction for the reactionenergy is -17.25+/-0.31 kcal/mol, setting an upper limit for the reactionenergies predicted by the post Hartree-Fock methods and a lower limit for theDFT reaction energies. We provide FN DMC input for clarifying the energeticuncertainties in the critical H-transfer reaction of syn-CH3CHOO. Thequantitatively close agreements between the FN DMC barrier height andexperimental measurement, and between the predictions from the FN DMC and G3model for the reaction energy provide a theoretical basis for resolving theenergy uncertainty in this reaction.

Quantum Monte Carlo Investigation of the H-transfer Reaction of Criegee Intermediate CH3CHOO: A Benchmark Calculation