  Heavy-fermion family exhibits fascinating and often puzzling properties due
to the presence of open-shell $f$ ions and the complexity of the associated
charge, orbital, and spin degrees of freedom. SmB$_6 $ is a prototypical
heavy-fermion compound that is electrically insulating but yet it displays
quantum oscillations, which are a telltale signature of the metallic state.
Adding to the enigma is the possibility that SmB$_6$ is a topological Kondo
insulator. Here, by treating the spin degree of freedom on an equal footing
with other degrees of freedom using the parameter-free strongly-constrained and
appropriately-normed (SCAN) density functional, we explore the ground-state
electronic structure of SmB$_{6}$. A number of competing magnetic phases lying
very closely in energy are found, indicating the key role of spin fluctuations
in the material. The computed band structure, crystal-field splittings in the
$f$-electron complex, the heavy effective electron mass at the Fermi energy,
and the large specific heat are all in good agreement with the corresponding
experimental results. In particular, our predicted FS explains the
experimentally observed bulk quantum oscillations as well as the low electrical
conductivity of SmB$_{6}$. The topological Kondo state of SmB$_6$ is shown to
be robust regardless of its magnetic configuration. The excellent performance
of SCAN in heavy-fermion systems is explained in terms of its ability to treat
self-interaction errors and symmetry breaking within the framework of the
density functional theory. Our study provides a new approach for modeling
heavy-fermion materials.
