Ab initio study of pressure-induced structural and electronic phase transitions in Ca 2 RuO 4

${\mathrm{Ca}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ is a compound with a Mott insulating ground state, which responds to external pressure in a variety of ways, some of which are expected (an insulator-metal transition) while others are more surprising (an expansion of the $c$ lattice constant). We provide here a comprehensive study of these pressure-induced structural and electronic changes using DFT$+U$ calculations, and demonstrate generally good agreement with experiment. The insulator-metal transition is reproduced and coincides with an isostructural transition, the $c$ lattice expansion, $\mathrm{Ru}{\mathrm{O}}_{6}$ octahedral distortions, and associated changes to the Ru $4d$ orbital order. The metallic part of the phase diagram features several competing phases. The high-pressure $Bbcm$ phase is found to be unstable in the ground state over a broad pressure range and suggested to be thermally stabilized instead.