  Using evolutionary algorithm for crystal structure prediction, we present a
new stable two-dimensional (2D) carbon allotrope composed of polymerized
as-indacenes (PAI) in a zigzag pattern, namely PAI-graphene whose energy is
lower than most of the reported 2D allotropes of graphene. Crucially, the
crystal structure realizes a nonsymmorphic layer group that enforces a
nontrivial global topology of the band structure with two Dirac cones lying
perfectly at the Fermi level. The absence of electron/hole pockets makes
PAI-graphene a pristine crystalline topological semimetal having anisotropic
Fermi velocities with a high value of $7.0 \times 10^{5}$ m/s. We show that
while the semimetallic property of the allotrope is robust against the
application of strain, the positions of the Dirac cone and the Fermi velocities
can be modified significantly with strain. Moreover, by combining strain along
both the x- and y-directions, two band inversions take place at $\Gamma$
leading to the annihilation of the Dirac nodes demonstrating the possibility of
strain-controlled conversion of a topological semimetal into a semiconductor.
Finally we formulate the bulk-boundary correspondence of the topological nodal
phase in the form of a generalized Zak-phase argument finding a perfect
agreement with the topological edge states computed for different
edge-terminations.
