A comparative charge density study on a series of estrogen derivatives has been initiated. The study utilizes the Hansen–Coppens atom-centred multipole model to describe the valence electron density distribution. Direct comparison of the population parameters for each estrogen after the respective multipole refinements requires standardization of the atom-centred local coordinate systems. Such a standard coordinate system for the common estrogen core is reported, taking advantage of the shape of those multipoles which have the spatial characteristics ofsp2andsp3hybrid orbitals. Additionally, populating these principal multipoles at the beginning stage of the refinements improves the stability of these large highly correlated calculations.
