  The electronic structures and topological properties of the orthorhombic and
monoclinic phases of the quasi-one-dimensional excitonic insulator Ta2NiSe5 are
investigated based on density functional theory. In contrast to a single parity
or band inversion across the Fermi level in many topological insulators studied
previously, there are multiple parity and band inversions with or without
spin-orbit coupling in both phases of Ta2NiSe5, resulting in more complex and
topologically nontrivial electronic structures. The Dirac cone type surface
states of the low-temperature monoclinic phase are also obtained. In this
paper, we demonstrate that Ta2NiSe5 is a promising candidate as a
three-dimensional topological excitonic insulator.
