The hydroxy peroxy radical derived from the oxidation of 3-methyl-3-buten-1-ol (MBO331), can undergo four different hydrogen shift (H-shift) reactions. We have compared optimized geometries, barrier heights and reaction rate constants obtained with five different DFT functionals (BLYP, B3LYP, BHandHLYP, wB97X-D and M06-2X) with the aug-cc-pVTZ basis set. We found that the single-point CCSD(T)-F12A/VDZ-F12 energies calculated at the different DFT geometries had very similar barrier heights. The wB97X-D, M06-2X and CCSD(T)-F12A/VDZ-F12 barrier heights are comparable. The atmospheric decomposition of the MBO331 peroxy radical was found to undergo a 1,5-CH H-shift reaction with a reaction rate constant of about 1 s^-1.