  A method is proposed for obtaining a systematic expansion of thermodynamic
functions of spin systems with large spin S in powers of 1/S. It uses the
cumulant technique and a coherent-state representation of the partition
function Z. The expansion of Z in terms of cumulants yields an effective
classical Hamiltonian with temperature-dependent quantum corrections. For the
Heisenberg quantum Hamiltonian, they have a non-Heisenberg form. The effective
Hamiltonian can be solved by methods familiar for classical systems.
