  In a recent letter [J. -H. Choi et al. Phys. Rev. Lett. 115, 066403 (2015)],
a universal linear relation between the binding energy Eb of exciton and the
band gap Eg is found in different quasi-2D semiconductors. However, when one
extrapolates the straight line to Eg=0, a positive Eb is obtained. This means
that a stable exciton exists in a metal (Eg=0), or in other words, the
excitation energy is negative. Therefore, for quasi-2D semiconductors with
small band gap, Eb vs. Eg must deviate from the straight line suggested in the
Letter.
  We performed similar GW+BSE calculation as the Letter for compressed
phosphorene and stretched graphene. At larger band gap, our data fall in the
same straight lien as the Letter. But for the quasi-2D semiconductors with
small band gap, the Eb vs. Eg curve approaches to zero. A simple model based on
local instantaneous screened interaction gives reasonable agreement with the
more rigorous calculations. For the quasi-2D semiconductors with diminishing
gap Eg<0.3eV, due to the nonlocal retarded effect induced by the strong
screening in the small gap materials, one has to use 2-time transition
amplitude (Bethe-Salpeter wave function) to describe exciton.
