We propose a method for obtaining effective classical Hamiltonians \cal H for
many-body quantum spin systems with large spins. This method uses the
coherent-state representation of the partition function Z and the cumulant
expansion in powers of 1/S. For the quantum Hamiltonian \hat H of a Heisenberg
form, the 1/S corrections in \cal H have a non-Heisenberg many-spin form. The
effective Hamiltonian \cal H can be treated by methods familiar for classical
systems. The non-Heisenberg terms in \cal H may be responsible for such effects
as spin-Peierls transition and uplifting of the classical degeneracy by quantum
fluctuations.Comment: 8 Pages, 2 Figures, submitted to EPJ
