  We perform the fixed-node diffuse Monte Carlo (FN DMC) calculations to
determine the barrier height and reaction energy of a critical reaction, the
H-transfer reaction from syn-CH3CHOO to vinyl hydroperoxide. The FN DMC barrier
height is found to be 16.60+/-0.35 kcal/mol which agrees well with the
experimental measurement within a few tenths of kcal, justifying the
reliability of the FN DMC method for predicting barrier height of the rapid
unimolecular reaction of Criegee intermediates. By comparing the predictions
from the CCSD(t), G3 (MCG3), DFT and MP2 methods with respect to the FN DMC
results and available experiment measurement, we found that the CCSD(t) barrier
heights agree with the FN DMC counterpart within statistical errors, and is
within a closer agreement with experiment and FN DMC prediction than the
G3(MCG3) models. Barrier heights predicted from the relatively more economic
DFT methods are within a few tenths kcal of the FN DMC prediction. MP2 method
severely underestimates the barrier height. FN DMC prediction for the reaction
energy is -17.25+/-0.31 kcal/mol, setting an upper limit for the reaction
energies predicted by the post Hartree-Fock methods and a lower limit for the
DFT reaction energies. We provide FN DMC input for clarifying the energetic
uncertainties in the critical H-transfer reaction of syn-CH3CHOO. The
quantitatively close agreements between the FN DMC barrier height and
experimental measurement, and between the predictions from the FN DMC and G3
model for the reaction energy provide a theoretical basis for resolving the
energy uncertainty in this reaction.
