  A low-energy enhancement (LEE) has been observed in the deexcitation
$\gamma$-ray strength function ($\gamma$SF) of compound nuclei. The LEE has
been a subject of intense experimental and theoretical interest since its
discovery, and, if the LEE persists in heavy neutron-rich nuclei, it would have
significant effects on calculations of r-process nucleosynthesis. Standard
configuration-interaction (CI) shell-model calculations in medium-mass nuclei
have attributed the LEE to the magnetic dipole $\gamma$SF but such calculations
are computationally intractable in heavy nuclei. We review a combination of
beyond-mean-field many-body methods within the framework of the CI shell model
that enables the calculation of $\gamma$SF in heavy nuclei, and discuss the
recent theoretical identification of a LEE in the magnetic dipole $\gamma$SF of
lanthanide isotopes.
