  We consider an arbitrary translationally invariant chain model with nearest
neighbors interaction and satisfying periodic boundary condition.
  The approach developed here allows a thermodynamic description of the chain
model directly in terms of grand potential per site. This thermodynamic
function is derived from an auxiliary function constructed only from open
connected sub-chains. In order to exemplify its application and how this
approach works we consider the Heisenberg XXZ model. We obtain the coefficients
of the high temperature expansion of the free energy per site of the model up
to third order.
