Approach to the spin-1/2 quantum Heisenberg model

An evolution equation for the expectation values of the Boltzmann factor between monomer-valence-bond states is derived. It contains the whole information on the thermodynamical and magnetic properties of the spin-1/2 quantum Heisenberg model. A new approximation scheme is proposed and studied in the thermodynamical limit. To lowest order, it leads to explicit formulas of the internal energy for the antiferromagnetic Heisenberg model in d dimensions.