  Calculating the quasiparticle (QP) band structure of two-dimensional (2D)
materials within the GW self-energy approximation has proven to be a rather
demanding computational task. The main reason is the strong
$\mathbf{q}$-dependence of the 2D dielectric function around $\mathbf{q} =
\mathbf{0}$ that calls for a much denser sampling of the Brillouin zone than is
necessary for similar 3D solids. Here we use an analytical expression for the
small $\mathbf{q}$-limit of the 2D response function to perform the BZ integral
over the critical region around $\mathbf{q} = \mathbf{0}$. This drastically
reduces the requirements on the $\mathbf{q}$-point mesh and implies a
significant computational speed-up. For example, in the case of monolayer
MoS$_2$, convergence of the $G_0W_0$ band gap to within $\sim 0.1\,\mathrm{eV}$
is achieved with $12\times 12$ $\mathbf{q}$-points rather than the $36\times
36$ mesh required with discrete BZ sampling techniques. We perform a critical
assessment of the band gap of the three prototypical 2D semiconductors MoS$_2$,
hBN, and phosphorene including the effect of self-consistency at the GW$_0$ and
GW level. The method is implemented in the open source GPAW code.
