The optimised structures of the conventional unit cells of CTO, STO and BTO are shown in Figure . The ABX 3 structure comprises a network of corner-sharing BX 6 octahedra surrounding cuboctahedral cavities filled by the A-site cations. The ideal perovskite structure is cubic with the Pm3m spacegroup, but this often not the lowest-energy configuration. Instead, many perovskites adopt one of a rich variety of structures obtained by rigid tilting of the octahedra and/or by displacements of the A-or B-site cations from their ideal positions in the cubic phase. In this case, the higher-symmetry rhombohedral R3m structure of BTO is derived from the cubic aristotype by a displacement of the Bsite cation along the [111] direction, while the lower-symmetry tetragonal I4/mcm structure of STO is derived by a single octahedral tilt and the orthorhombic Pnma structure of CTO is derived by three independent tilts. The optimised lattice parameters are collected in Table and compared to experimental measurements from the literature. The lattice parameters of CTO, STO and BTO are within 2%, 1% and 0.5%, respectively, of the experimental measurements. These "athermal" optimised structures are those 0 K without corrections for the vibrational zero-point energy, whereas experimental measurements are performed at finite temperature and may include thermal changes to the lattice constants, so we consider these small differences acceptable. Our optimised lattice parameters are also within 5% of those of the initial MP structures, although these are optimised with the PBE functional, which typically overestimates lattice parameters and unit-cell volumes relative to experiments.