  The growing interest in tin-halide semiconductors for photovoltaic
applications demands an in-depth knowledge of the fundamental properties of its
constituents, starting from the smallest monomers entering the initial stages
of formation. In this first-principles work based on time-dependent
density-functional theory, we investigate the structural, electronic, and
optical properties of tin-halide molecules SnX$_n^{2-n}$, with $n=1,2,3,4$ and
X = Cl, Br, I, simulating these compounds in vacuo as well as in an implicit
solvent. We find that structural properties are very sensitive to the halogen
species while the charge distribution is also affected by stoichiometry. The
ionicity of the Sn-X bond is confirmed by the Bader charge analysis albeit
charge displacement plots point to more complex metal-halide coordination.
Particular focus is posed on the neutral molecules SnX$_2$, for which
electronic and optical properties are discussed in detail. Band gaps and
absorption onset decrease with increasing size of the halogen species and
despite general common features, each molecule displays peculiar optical
signatures. Our results are elaborated in the context of experimental and
theoretical literature, including the more widely studied lead-halide analogs,
aiming to contribute with microscopic insight to a better understanding of
tin-halide perovskites.
