  A primary goal at the interface of theoretical and experimental quantum
magnetism is the investigation of exotic spin states, most notably quantum spin
liquids (QSLs). Magnetic rare-earth ions go beyond the straightforward paradigm
of geometrical frustration in Heisenberg antiferromagnets by introducing
competing energy scales, and in particular their strong spin-orbit coupling
creates multiple split crystal electric-field (CEF) levels, leading to
anisotropic effective spin models with intrinsic frustration. While rare-earth
delafossites have a triangular-lattice geometry, and thus have gained recent
attention as candidates for hosting spin-1/2 QSL physics, the reliable
extraction of effective spin models from the initial many-parameter CEF
spectrum is a hard problem. Using the example of CsYbSe2, we demonstrate the
unambiguous extraction of the Stevens-operators coefficients dictating the full
CEF spectrum of Yb$^{3+}$ by translating these into parameters with a direct
physical interpretation. Specifically, we combine low-field susceptibility
measurements with resonant torsion magnetometry experiments in fields up to 60
T to determine a sufficiently large number of physical parameters -- effective
Zeeman splittings, anisotropic van Vleck coefficients, and magnetotropic
coefficients -- that the set of Stevens-operator coefficients is unique. Our
crucial identification of the strong corrections to the Zeeman splitting of
Kramers doublets as van Vleck coefficients has direct consequences for the
interpretation of all anisotropic magnetic susceptibility measurements. Our
results allow us to determine the nature and validity of an effective spin-1/2
model for CsYbSe2, to provide input for theoretical studies of such models on
the triangular lattice, and to provide additional materials insight into routes
for achieving magnetic frustration and candidate QSL systems in rare-earth
compounds.
