  The pair cluster (dimer) is studied within the framework of the extended
Hubbard model and the grand canonical ensemble. The elastic interatomic
interactions and thermal vibrational energy of the atoms are taken into
account. The total grand potential is constructed, from which the equation of
state is derived. In equilibrium state, the deformation of cluster size, as
well as its derivatives, are studied as a function of the temperature and the
external magnetic and electric fields. In particular, the thermal expansion,
magnetostriction and electrostriction effects are examined for arbitrary
temperature, in a wide range of Hamiltonian parameters.
