  The electronic properties of two-dimensional transition metal dichalcogenides
(2D TMDs) have attracted much attention during the last decade. We show how a
diagrammatic ab initio coupled cluster singles and doubles (CCSD) treatment
paired with a careful thermodynamic limit extrapolation in two dimensions can
be used to obtain converged bandgaps for monolayer materials in the MoS2
family. We find general agreement between CCSD and previously reported GW
simulations in terms of the band structure, but predict slightly higher band
gap values and effective hole masses compared to previous reports. We also
investigate the ability of CCSD to describe trion states, finding reasonable
qualitative structure, but poor excitation energies due to the lack of
screening of three-particle excitations in the effective Hamiltonian. Our study
provides an independent high-level benchmark of the role of many-body effects
in 2D TMDs and showcases the potential strengths and weaknesses of diagrammatic
coupled cluster approaches for realistic materials.
