  We provide a first-principles description of the crystalline and
oxygen-deficient Ta2O5 using refined computational methods and models. By
performing calculations on a number of candidate structures, we determined the
low-temperature phase and several stable oxygen vacancy configurations, which
are notably different from the previous results. The most stable charge-neutral
vacancy site induces a shallow level near the bottom of conduction band.
Stability of different charge states is studied. Based on the results, we
discuss the implications of the level structures on experiments, including the
leakage current in Ta2O5-based electronic devices and catalysts.
