  The operator fidelity is a measure of the information-theoretic
distinguishability between perturbed and unperturbed evolutions. The response
of this measure to the perturbation may be formulated in terms of the operator
fidelity susceptibility (OFS), a quantity which has been used to investigate
the parameter spaces of quantum systems in order to discriminate their regular
and chaotic regimes. In this work we numerically study the OFS for a pair of
non-linearly coupled two-dimensional harmonic oscillators, a model which is
equivalent to that of a hydrogen atom in a uniform external magnetic field. We
show how the two terms of the OFS, being linked to the main properties that
differentiate regular from chaotic behavior, allow for the detection of this
model's transition between the two regimes. In addition, we find that the
parameter interval where perturbation theory applies is delimited from above by
a local minimum of one of the analyzed terms.
