The valence electrons were described explicitly using a plane-wave basis set with an energy cut-off of 450 eV. The valence electron configurations used for the chemical species are: W = 6s 1 5d 5 ; Cu = 4s 1 3d 10 ; S = 3s 2 3p 4 ; Se = 4s 2 4p 4 ; Te = 5s 2 5p 4 . The core electrons were described using the projector-augmented wave potential (PAW) . A Monkhorst-Pack k-point grid of 2×2×1 was used. All forces acting on the atoms were converged to within 0.02 eV/ Å, with convergence criteria for energy minimisation set to the default value of 10 -4 for geometry relaxations. A Methfessel-Paxton smear-ing of order 1 was used and no symmetry was applied. We use a Gaussian smearing of σ = 0.1 eV. A vacuum greater than 17 Å was applied to separate the repeating images for the monolayer along the z direction.