The optimised lattice parameters of the three Bi 2 ChO 2 are collected in Table . The calculations on all three structures agree very well with experiments, with a slight 0.5-2.3% underestimation of the lattice parameters that can be accounted for by the "athermal" DFT calculations being performed at 0 K without zero-point corrections. (We note that the Bi 2 SO 2 structure reported in Ref. is in the P nma spacegroup, but has a very similar structure to Bi 2 SeO 2 and Bi 2 TeO 2 , and the tetragonal I4/mmm spacegroup can be recovered with a loose symmetry tolerance of 1 Å.) The predicted lattice constants and cell volumes increase in the order Bi 2 SO 2 < Bi 2 SeO 2 < Bi 2 TeO 2 , as expected given the increase in atomic radii from 100-140 pm, and this trend is also present in the experimental structures.