The crystal structure of CaCuP is shown in Figure ,b, and is well reproduced by DFT. It consists of trigonal planar Cu atoms (dark orange) coordinated to P atoms (lilac) in the ab-plane, with Ca atoms (pale orange) sitting above the hexagonal voids in the cdirection. Table † shows the lattice parameters obtained after structure optimisation using various exchange-correlation functionals, as well as a comparison to experiment and previous DFT calculations. An extended discussion of the crystal structureelectronic structure relationship in CaCuP can be found in Ref. A phonon dispersion curve is produced from lattice-dynamics calculations using a converged 6 × 6 × 6 q-point grid in Figure .