  First-principles simulations have been performed to investigate the phase
stability of tantalum metal under high pressure and high temperature (HPHT). We
searched its low-energy structures globally using our developed multi-algorithm
collaborative (MAC) crystal structure prediction technique. The body-centred
cubic (bcc) was found to be stable at pressure up to 300 GPa. The previously
reported $\omega$ and \textit{A}15 structures were also reproduced
successfully. More interestingly, we observed another phase (space group:
\textit{Pnma}, 62) that is more stable than $\omega$ and \textit{A}15. Its
stability is confirmed by its phonon spectra and elastic constants. For
$\omega$-Ta, the calculated elastic constants and high-temperature phonon
spectra both imply that it is neither mechanically nor dynamically stable.
Thus, $\omega$ is not the structure to which bcc-Ta transits before melting. On
the contrary, the good agreement of \textit{Pnma}-Ta shear sound velocities
with experiment suggests \textit{Pnma} is the new structure of Ta implied by
the discontinuation of shear sound velocities in recent shock experiment [J.
Appl. Phys. \textbf{111}, 033511 (2012)].
