  We show that in one-dimensional isotropic Heisenberg model two-qubit thermal
entanglement and maximal violation of Bell inequalities are directly related
with a thermodynamical state function, i.e., the internal energy. Therefore
they are completely determined by the partition function, the central object of
thermodynamics. For ferromagnetic ring we prove that there is no thermal
entanglement at any temperature. Explicit relations between the concurrence and
the measure of maximal Bell inequality violation are given.
