  We suggest a tetragonal $I4_1/amd$ phase ($\eta$-phase) as the ground state
of $\mathrm{Ta_2O_5}$ at zero temperature, which is a high symmetry version of
the triclinic $\gamma$-phase $\mathrm{Ta_2O_5}$ predicted by Yang and Kawazoe.
Our calculation shows that $\gamma$-phase $\mathrm{Ta_2O_5}$ will automatically
be transformed into the $\eta$-phase during structural relaxation. Phonon
dispersion confirms that the $\eta$-phase is dynamically stable, while the high
temperature $\alpha$-phase $\mathrm{Ta_2O_5}$, which also has the $I4_1/amd$
symmetry, is unstable at zero temperature. A thorough energy comparison of the
$\beta_{AL}$, $\delta$, $\lambda$, $\mathrm{B}$, $\mathrm{L_{SR}}$, $\beta_R$,
$Pm$, $Cmmm$, $\gamma$, $\eta$ and $\alpha$ phases of $\mathrm{Ta_2O_5}$ is
carried out. The GGA-1/2 method is applied in calculating the electronic
structure of various phases, where the $\eta$-phase demonstrates a 4.24 eV
indirect band gap, close to experimental value. The high symmetry tetragonal
phase together with computationally efficient GGA-1/2 method greatly facilitate
the $ab\ initio$ simulation of $\mathrm{Ta_2O_5}$-based devices. As an example,
we have explicitly shown the Ohmic contact nature between metal Ta and
$\mathrm{Ta_2O_5}$ by calculating an interface model of $b.c.c.$ Ta and
$\eta$-$\mathrm{Ta_2O_5}$, using GGA-1/2.
