Quasiclassical Hamiltonians for large-spin systems

We extend and apply a previously developed method for a semiclassical treat ment of a system with large spin S. A multisite Heisenberg Hamiltonian is t ransformed into an effective classical Hamilton function which can be treat ed by standard methods for classical systems. Quantum effects enter in form of multispin interactions in the Hamilton function. The latter is written in the form of an expansion in powers of J/(TS), where J is the coupling co nstant. Main ingredients of our method are spin coherent states and cumulan ts. Rules and diagrams are derived for computing cumulants of groups of ope rators entering the Hamiltonian. The theory is illustrated by calculating t he quantum corrections to the free energy of a Heisenberg chain which were previously computed by a Wigner-Kirkwood expansion.