AtomAccess is written in Python (≥ v3.6), is open source 40 and licensed under GNU GPLv3, and is available on the PyPI package repository; the program is scriptable and useable on both command-line and web interfaces (Figure ), where the latter allows interactive visualization (Figure ). AtomAccess quantifies the accessibility of an atom in a molecule via the size of the largest binding site, which is calculated as the largest continuous fraction of solid angle that is not obscured by surrounding atoms. The selected atom is treated as a point source of light, from which a collection of rays emanate on a Zaremba-Conroy-Wolfsberg (ZCW) angular grid (with density index ρ as described by Levitt ), evenly sampling points on the surface of a sphere. Intersections of the ray vectors with the surrounding atoms (spheres defined with van der Waals atomic radii ) are calculated up to a radial cutoff (rmax) from the central atom; here we set rmax to encompass the entire molecule. Unblocked rays represent points on the surface of a sphere of radius rmax from the central atom and can be clustered with a singlelinkage agglomerative clustering algorithm in Scikit-learn (see ESI for details), such that adjacent points are clustered together (Figure ). The size of a potential binding site is calculated as the number of unblocked rays in a cluster as a percentage of the total number of rays, which for the ZCW angular grid is equivalent to the percentage solid angle (% SA) subtended by that cluster. AtomAccess reports the total percentage of unblocked rays (comparable to 100-G in Solid-G), together with the sizes of all clusters. In our experience, the size of the largest cluster is a good metric for assessing the accessibility of a metal complex;