Left, harmonic and anharmonic phonon spectrum, maintaining a 62.3° rhombohedral angle. The harmonic calculation is performed with the internal atomic positions that yield classical vanishing forces. The anharmonic calculation is performed after relaxing (with the SSCHA) the internal degrees of freedom but maintaining the 62.3° rhombohedral angle. At the harmonic level there are unstable phonon modes even at Γ. Symmetry prevents the relaxation of this structure according to the unstable phonon mode at Γ. The harmonic phonons are calculated at a classic pressure of 150 GPa. Quantum effects add around an extra 10 GPa to the pressure. To the right of the graph is shown the behaviour of λ(ω) and α2F(ω) for the anharmonic calculation. Right, pressure along the different Cartesian directions during the SSCHA relaxation of the internal parameters, keeping the rhombohedral angle fixed at 62.3°. At step 0 the pressure reported is obtained directly from V(R), neglecting quantum effects. It is isotropic within 1 GPa of difference between the x–y and z directions. At each of the other steps it is calculated from the quantum \(E({\boldsymbol{ {\mathcal R} }})\) and along the minimization it becomes anisotropic. When the minimization stops at step 12—that is, the internal coordinates are at the minimum of the \(E({\boldsymbol{ {\mathcal R} }})\) for this lattice—the stress anisotropy between the z and the x–y directions is about 6%. This clearly indicates that quantum effects act to relax the crystal lattice—in particular, because P z is larger—by reducing the rhombohedral angle. It is worth noting that quantum effects increase the total pressure by approximately 10 GPa, which is calculated as P = (P x + P y + P z )/3. The initial cell parameters before the minimization are a = 3.5473398 Å and α = 62.34158°. The initial values of the free Wyckoff parameters, which yield classical vanishing forces and a 150-GPa isotropic stress, are ε a = 0.26043, ε b = 0.09950, ε x = 0.10746 and ε y = 0.12810. See Extended Data Table 2 for more details.