  The electronic properties of single-layer antimony are studied by a
combination of first-principles and tight-binding methods. The band structure
obtained from relativistic density functional theory is used to derive an
analytic tight-binding model that offers an efficient and accurate description
of single-particle electronic states in a wide spectral region up to the
mid-UV. The strong ($\lambda=0.34$ eV) intra-atomic spin-orbit interaction
plays a fundamental role in the band structure, leading to splitting of the
valence band edge and to a significant reduction of the effective mass of the
hole carriers. To obtain an effective many-body model of two-dimensional Sb we
calculate the screened Coulomb interaction and provide numerical values for the
on-site $\bar{V}_{00}$ (Hubbard) and intersite $\bar{V}_{ij}$ interactions. We
find that the screening effects originate predominantly from the 5$p$ states,
and are thus fully captured within the proposed tight-binding model. The
leading kinetic and Coulomb energies are shown to be comparable in magnitude,
$|t_{01}|/(\bar{V}_{00}-\bar{V}_{01}) \sim 1.6$, which suggests a strongly
correlated character of 5$p$ electrons in Sb. The results presented here
provide an essential step toward the understanding and rational description of
a variety of electronic properties of this two-dimensional material.
