DFT-D geometry optimizations were performed using a two step process of first optimizing atomic positions with fixed lattice parameters, before then optimizing all degrees of freedom; this has been found to improve convergence for molecular crystals . All calculations used a plane-wave energy cutoff of 500 eV, using the projector-augmented wave (PAW) method and standard pseudopotentials . Tolerances for energy convergence were set to 10 -7 eV per atom, and force tolerances for geometry optimization to 3 × 10 -2 eV/Å.