Theoretical predictions suggest carbon dioxide phases III and VII are identical† †Electronic supplementary information (ESI) available: Comparisons between predicted and experimental structures, bulk modulus data, assessment of modeling choices on the predicted Raman spectra, pressure-dependent Raman data, and crystal structure prediction results. See DOI: 10.1039/c7sc03267f Click here for additional data file. Click here for additional data file. Click here for additional data file.

Solid carbon dioxide exhibits a rich phase diagram at high pressures. Metastable phase III is formed by compressing dry ice above 10–12 GPa. Phase VII occurs at similar pressures but higher temperatures, and its stability region is disconnected from III on the phase diagram. Comparison of large-basis-set quasi-harmonic second-order Møller–Plesset perturbation theory calculations and experiment suggests that the long-accepted structure of phase III is problematic. The experimental phase III and VII structures both relax to the same phase VII structure. Furthermore, Raman spectra predicted for phase VII are in good agreement with those observed experimentally for both phase III and VII, while those for the purported phase III structure agree poorly with experimental observations. Crystal structure prediction is employed to search for other potential structures which might account for phase III, but none are found. Together, these results suggest that phases III and VII are likely identical.