  We investigated the effects of paramagnetic (PM) fluctuations on the
thermochemistry of the MnO(100) surface in the oxygen evolution reaction (OER)
using the "noncollinear magnetic sampling method \textit{plus} $U$"
(NCMSM$+U$). Various physical properties, such as the electronic structure,
free energy, and charge occupation, of the MnO (100) surface in the PM state
with several OER intermediates, were reckoned and compared to those in the
antiferromagnetic (AFM) state. We found that PM fluctuation enhances charge
transfer from a surface Mn ion to each of the intermediates and strengthens the
chemical bond between them, while not altering the overall features, such as
the rate determining step and resting state, in reaction pathways. The enhanced
charge transfer can be attributed to the delocalized nature of valence bands
observed in the PM surface. In addition, it was observed that chemical-bond
enhancement depends on the intermediates, resulting in significant deviations
in reaction energy barriers. Our study suggests that PM fluctuations play a
significant role in the thermochemistry of chemical reactions occurring on
correlated oxide surfaces.
