  Alkali metal rare-earth chalcogenide $ARECh2$ (A=alkali or monovalent metal,
RE=rare earth, Ch=O, S, Se, Te), is a large family of quantum spin liquid (QSL)
candidates we discovered recently. Unlike $YbMgGaO4$, most members in the
family except for the oxide ones, have relatively small crystalline
electric-field (CEF) excitation levels, particularly the first ones. This makes
the conventional Curie-Weiss analysis at finite temperatures inapplicable and
CEF excitations may play an essential role in understanding the low-energy spin
physics. Here we considered an effective magnetic Hamiltonian incorporating CEF
excitations and spin-spin interactions, to accurately describe thermodynamics
in such a system. By taking $NaYbSe2$ as an example, we were able to analyze
magnetic susceptibility, magnetization under pulsed high fields and heat
capacity in a systematic and comprehensive way. The analysis allows us to
produce accurate anisotropic exchange coupling energies and unambiguously
determine a crossover temperature ($\sim$25 K in the case of $NaYbSe2$), below
which CEF effects fade away and pure spin-spin interactions stand out. We
further validated the effective picture by successfully explaining the
anomalous temperature dependence of electron spin resonance (ESR) spectral
width. The effective scenario in principle can be generalized to other
rare-earth spin systems with small CEF excitations.
