The article of record as published may be found at https://doi.org/10.1063/1.476144We show that the measured magnetic susceptibility of molecular ring clusters can be accurately reproduced, for all but low temperatures T, by a classical Heisenberg model of N identical spins S on a ring that interact with isotropic nearest-neighbor interactions. While exact expressions for the two-spin correlation function, C{sub N}(n,T), and the zero-field magnetic susceptibility, {chi}{sub N}(T), are known for the classical Heisenberg ring, their evaluation involves summing infinite series of modified spherical Bessel functions. By contrast, the formula C{sub N}(n,T)=(u{sup n}+u{sup N{minus}n})/(1+u{sup N}), where u(K)=cothK{minus}K{sup {minus}1} is the Langevin function and K=JS(S+1)/(k{sub B}T) is the nearest-neighbor dimensionless coupling constant, provides an excellent approximation if N{ge}6 for the regime {vert_bar}K{vert_bar}{lt}3. This choice of approximant combines the expected exponential decay of correlations for increasing yet small values of n, with the cyclic boundary condition for a finite ring, C{sub N}(n,T)=C{sub N}(N{minus}n,T). By way of illustration, we show that, for T{gt}50K, the associated approximant for the susceptibility derived from the approximate correlation function is virtually indistinguishable from both the exact theoretical susceptibility and the experimental data for the {open_quotes}ferric wheel{close_quotes} molecular cluster ([Fe(OCH{sub 3}){sub 2}(O{sub 2}CCH{sub 2}Cl)]{sub 10}), which contains N=10 interacting Fe{sup 3+} ions, each of spin S=5/2, that are symmetrically positioned in a nearly planar ring. {copyright} {ital 1998 American Institute of Physics.
