Optimal truncations of asymptotic expansions are known to yield
approximations to adiabatic quantum evolutions that are accurate up to
exponentially small errors. In this paper, we rigorously determine the leading
order non--adiabatic corrections to these approximations for a particular
family of two--level analytic Hamiltonian functions. Our results capture the
time development of the exponentially small transition that takes place between
optimal states by means of a particular switching function. Our results confirm
the physics predictions of Sir Michael Berry in the sense that the switching
function for this family of Hamiltonians has the form that he argues is
universal.