  Using first-principles calculations, the elastic constants, thermodynamic
property and structural phase transition of rhenium diboride under pressure are
investigated by means of the pseudopotential plane-waves method, as well as the
effect of metallic bond on its hardness. Eight candidate structures of known
transition-metal compounds are chosen to probe for rhenium diboride ReB2. The
calculated lattice parameters are consistent with the experimental and
theoretical values. Based on the third order equation of states, the transition
pressure Pt between the ReB2-ReB2 and MoB2-ReB2 phases is firstly determinate.
Elastic constants, shear modulus, modulus, ratio and temperature are derived.
The single-bonded feather remains in ReB2 compounds.Furthermore, a
semiempirical method to evaluate the hardness of multicomponent crystals with
partial metallic bond is presented. The effect of metallic bond on the hardness
of ReB2 is searched and the hardness shows a gradual increase under
compression, which is different from diamond.
