The electronic band structure of (NH 3 (CH 2 ) 7 NH 3 ) 2 Sn 3 I 10 resembles that of aristotypical ASnI 3 perovskites but with additional flat bands near the frontier electronic states. Relaxation of the experimental crystal structure using hybrid functional DFT calculations with explicit inclusion of spin-orbit coupling (HSE06+SOC) does not significantly alter the structure, as comparison with the experimental structure yields a similarity metric of δ = 0.16. The largest atom position differences occur in the H atom positions; the largest shifts of the inorganic atom positions result from I(5) (0.3707 Å) and I(4), I(6) (0.21 Å); these all point toward the vacant tin site. All other shifts are lower than 0.2 Å (see Table ). The complete description of the relaxed structure is provided in Table .