Abstract
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 
.
The expansion of 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.