  In the framework of classical statistical mechanics and assuming the
electronic ground-state Born-Oppenheimer approximation, we show in this work
that a dependence of the equilibrium bond-length on the available volume is to
be expected for a dilute gas of diatomic molecules. In a nutshell, this
dependence is controlled by the relation between the potential well depth $D$
(through the factor $e^{-\beta D}$) and the quotient $L/R$ of the linear size
of the container $L$ and the potential well width $R$. Using simplified
analytical estimations, we predict that the equilibrium bond-length $\langle
\rho \rangle$ is independent of $L$ for a range of volumes which is
exponentially large on $\beta D$. At some point of the $L$ axis, $\langle \rho
\rangle$ starts to increase and it eventually diverges, as it is to be
expected, when $L \to \infty$, thus describing dissociated atoms. According to
our estimations, it is possible that some diatomic molecules, such as halogens,
could present volume dependence of their equilibrium bond-lengths for
laboratory-size volumes at room temperature.
