Cu 2 SiSe 3 belongs to the adamantine family of materials, crystallizing into the monoclinic Cc space group. There are two inequivalent Cu I sites and one Si IV site, each connected to four Se II anions to form corner-sharing tetrahedra, Figure . The lattice parameters of the conventional unit cell, determined using the HSE06 functional, are a = 6.70 Å (0.4 %), b = 11.84 Å (0.3 %), c = 6.67 Å (0.6 %), with percentage errors given with respect to experimental values. Lattice parameters calculated using other DFT functionals are provided in the Supporting Information Section 1.