  At non-zero temperature and when a system has low-lying excited electronic
states, the ground-state Born--Oppenheimer approximation breaks down and the
low-lying electronic states are involved in any chemical process. In this work,
we use a temperature-dependent effective potential for the nuclei which can
accomodate the influence of an arbitrary number of electronic states in a
simple way, while at the same time producing the correct Boltzmann equibrium
distribution for the electronic part. With the help of this effective
potential, we show that thermally-activated low-lying electronic states can
have a significant effect in molecular properties for which electronic
excitations are oftentimes ignored. We study the thermal expansion of the
Manganese dimer, Mn$_2$, where we find that the average bond length experiences
a change larger than the present experimental accuracy upon the inclusion of
the excited states into the picture. We also show that, when these states are
taken into account, reaction rate constants are modified. In particular, we
study the opening of the ozone molecule, O$_3$, and show that in this case the
rate is modified as much as a 20% with respect to the ground-state
Born--Oppenheimer prediction.
