  We investigate atomic and electronic structures of monolayer 1T-TaSe2 using
density functional theory calculations. Monolayers of 1T-TaSe2 were recently
grown on graphene substrates and suggested as an intriguing Mott insulator
[Nat. Phys. 16, 218 (2020)]. However, the prevailing structural model for the
model system of 1T-TaS2, the cation-centered cluster of a David-star shape with
strong electron correlation, could not explain the characteristic and unusual
orbital splitting observed in scanning tunneling spectroscopy experiments. We
suggest an alternative structure model, an anion-centered cluster structure,
which can reproduce most of the unusual spectroscopic characteristics with
electron doping from the substrate without electron correlation. The unusual
spectroscopic features observed, thus, seems to indicate a simple and usual
band insulating state. This work indicates the importance of a large structural
degree of freedom given for a cluster Mott insulator.
