  The elastic, thermodynamic, and electronic properties of fluorite RuO_2 under
high pressure are investigated by plane-wave pseudopotential density functional
theory. The optimized lattice parameters, elastic constants, bulk modulus, and
shear modulus are consistent with other theoretical values. The phase
transition from modified fluorite-type to fluorite is 88 GPa (by localized
density approximation, LDA) or 115.5 GPa (by generalized gradient
approximation, GGA). The Young's modulus and Lam\'e's coefficients are also
studied under high pressure. The structure turned out to be stable for the
pressure up to 120 GPa by calculating elastic constants. In addition, the
thermodynamic properties, including the Debye temperature, heat capacity,
thermal expansion coefficient, Gr\"uneisen parameter, and Poisson's ratio, are
investigated. A small band gap is found in the electronic structure of fluorite
RuO_2 and the bandwidth increases with the pressure. Also, the present
mechanical and electronic properties demonstrate that the bonding nature is a
combination of covalent, ionic, and metallic contributions.
