Quantum-to-classical crossover in generalized spin systems: Temperature-dependent spin dynamics of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>FeI</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>

Simulating quantum spin systems at finite temperatures is an open challenge in many-body physics. This work studies the temperature-dependent spin dynamics of a pivotal compound, ${\mathrm{FeI}}_{2}$, to determine if universal quantum effects can be accounted for by a phenomenological renormalization of the dynamical spin structure factor $S(\mathbf{q},\ensuremath{\omega})$ measured by inelastic neutron scattering. Renormalization schemes based on the quantum-to-classical correspondence principle are commonly applied at low temperatures to the harmonic oscillators describing normal modes. However, it is not clear how to extend this renormalization to arbitrarily high temperatures. Here we introduce a temperature-dependent normalization of the classical moments, the magnitude of which is determined by imposing the quantum sum rule, e.g., $\ensuremath{\int}d\ensuremath{\omega}d\mathbf{q}S(\mathbf{q},\ensuremath{\omega})={N}_{S}S(S+1)$ for ${N}_{S}$ dipolar magnetic moments. We show that this simple renormalization scheme significantly improves the agreement between the calculated and measured $S(\mathbf{q},\ensuremath{\omega})$ for ${\mathrm{FeI}}_{2}$ at all temperatures. Due to the coupled dynamics of dipolar and quadrupolar moments in that material, this renormalization procedure is extended to classical theories based on SU(3) coherent states, and by extension, to any $\mathrm{SU}(N)$ coherent state representation of local multipolar moments.