The paper by S. Lebègue and O. Eriksson investigates the electronic structure of two-dimensional (2D) crystals MoS$_2$ and NbSe$_2$ using ab-initio calculations. They find that 2D MoS$_2$ is a semiconductor with a gap similar to its 3D counterpart, while NbSe$_2$ is a metal, consistent with its 3D form. The authors also computed the electronic structures of 2D hexagonal lattices of Si and Ge, comparing them to graphene. They find that 2D silicon has an electronic structure similar to graphene, with a linear dispersion around the K-point, while 2D germanium is a poor metal due to its metallic nature. The study highlights the potential of 2D silicon for interfacing with existing electronic devices, despite the challenges in synthesizing it in a graphene-like structure.The paper by S. Lebègue and O. Eriksson investigates the electronic structure of two-dimensional (2D) crystals MoS$_2$ and NbSe$_2$ using ab-initio calculations. They find that 2D MoS$_2$ is a semiconductor with a gap similar to its 3D counterpart, while NbSe$_2$ is a metal, consistent with its 3D form. The authors also computed the electronic structures of 2D hexagonal lattices of Si and Ge, comparing them to graphene. They find that 2D silicon has an electronic structure similar to graphene, with a linear dispersion around the K-point, while 2D germanium is a poor metal due to its metallic nature. The study highlights the potential of 2D silicon for interfacing with existing electronic devices, despite the challenges in synthesizing it in a graphene-like structure.