Dipole-dipole forces in the Heisenberg model: dependence of the free energy on sample shape

We discuss the dependence of the free energy on the shape of the external boundary of a sample for a Heisenberg model with dipolar forces. We prove the theorem that enables us to perform a restricted renormalization of the vertices of time-ordered diagrams, and find that the molecular field rigorously contains all the shape-dependent terms in the free energy.