  Parametric energy-level correlation describes the response of the
energy-level statistics to an external parameter such as the magnetic field.
Using semiclassical periodic-orbit theory for a chaotic system, we evaluate the
parametric energy-level correlation depending on the magnetic field difference.
The small-time expansion of the spectral form factor $K(\tau)$ is shown to be
in agreement with the prediction of parameter dependent random-matrix theory to
all orders in $\tau$.
