Full unit cell relaxation was performed with either CASTEP 118 or FHI-aims (which code was used is reported in the data records). The Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional 121 was used with the Tkatchenko-Scheffler (TS) pairwise dispersion method 122 . For relaxations performed with CASTEP, norm-conserving pseudopotentials were utilized for carbon and hydrogen. The plane-wave basis set cutoff was 750 eV. A Monkhorst-Pack k-grid with a spacing of 0.07 Å -1 was adopted. The convergence thresholds for total energy, maximum force, maximum stress, and maximum displacement were 5 × 10 -6 eV/atom, 0.01 eV/Å, 0.02 GPa, and 5 × 10 -4 Å -1 , respectively. For structures relaxed with FHI-aims, the tight numerical settings and tier-2 basis sets were used. The fully relaxed crystal structures and the molecular geometries extracted from them are provided in the data records. The GW+BSE calculations were performed for the fully relaxed crystal structures.