atomic electrons explicitly. We used the GGA-PBE functional [@Perdew_etal_1996_PRL_PBEFunctional; @Perdew_etal_1997_PRL_PBEFunctionalErrata] and CASTEP’s internal on-the-fly-generated pseudopotentials for Y, W and O, regenerated so as to be norm-conserving. The material was explicitly defined as an insulator for all calculations. Forces, stresses and dielectric constants were converged to within 0.005 eVÅ$^{-1}$, 0.01 GPa and 0.00025 respectively. This was achieved using a plane wave cutoff energy of 1200 eV and a $3\times 2\times 3$ Monkhorst-Pack [@Monkhorst_Pack_1976_PRB_MPGrids] grid of wave-vectors for the integration of electronic states across the Brillouin zone. Geometry optimisation at 0 GPa gave final structural parameters as detailed in . Agreement with experiment is reasonable although, as ever with the GGA model, the calculation slightly overestimates bond distances and lattice parameters. shows that the differences occur mostly in the range $\le1\%$ except for the $b$ lattice parameter (just under $2\%$ discrepancy).

Phonon frequencies were calculated using Density Functional Perturbation Theory (DFPT) [@Baroni_DFPT; @Refson_etal_2006_PhysRevB_DFPTPaper] with a $3\times 2\times 3$ Monkhorst-Pack [@Monkhorst_Pack_1976_PRB_MPGrids] grid that was offset to place one of the wave-vectors at the origin of reciprocal space. Fourier interpolation was used to calculate phonons for wave vectors along high-symmetry directions [@Aroyo_etal_2011_BilbaoCrystServer1; @Aroyo_etal_2006_ZKrist_BilbaoCrystServer1; @Aroyo_etal_2006_ActaCrystA_BilbaoCrystServer2; @Tasci_etal_2012_Conference_BilbaoCrystServerKVEC; @BilbaoCrystServer] for the production of dispersion curves. In addition, 