  The observed modification of thermal chemical rates in Fabry-Perot cavities
remains a poorly understood effect theoretically. Recent breakthroughs explain
some of the observations through the Grote-Hynes theory, where the cavity
introduces friction with the reaction coordinate, thus reducing the
transmission coefficient and the rate. The regime of rate enhancement, the
observed sharp resonances at varying cavity frequencies, and the survival of
these effects in the collective regime remain mostly unexplained. In this
paper, we consider the \emph{cis}-\emph{trans} isomerization of HONO
atomistically using an \emph{ab-initio} potential energy surface. We evaluate
the transmission coefficient using the reactive flux method and identify the
conditions for rate acceleration. In the underdamped, low-friction regime of
the reaction coordinate, the cavity coupling enhances the rate with increasing
coupling strength until reaching the Kramers turnover point. Sharp resonances
in this regime are related to cavity-enabled energy redistribution channels.
