It is known that the equilibrium properties of nanoscale systems can deviate
significantly from standard thermodynamics due to their coupling to an
environment. For the generalized θ-angled spin-boson model, here we
derive an explicit form of the classical mean force equilibrium state. Taking
the large spin limit of the quantum spin-boson model, we demonstrate that the
quantum-classical correspondence is maintained at arbitrary coupling strength.
This correspondence gives insight into the conditions for a quantum system to
be well-approximated by its classical counterpart. We further demonstrate that,
counterintuitively, previously identified environment-induced 'coherences' in
the equilibrium state of weakly coupled quantum spins, do not disappear in the
classical case. Finally, we categorise various coupling regimes, from
ultra-weak to ultra-strong, and find that the same value of coupling strength
can either be 'weak' or 'strong', depending on whether the system is quantum or
classical. Our results shed light on the interplay of quantum and mean force
corrections in equilibrium states of the spin-boson model, and will help draw
the quantum to classical boundary in a range of fields, such as magnetism and
exciton dynamics