On the accuracy and precision of X-ray and neutron diffraction results as a function of resolution and the electron density model

X-ray diffraction is the main source of three-dimensional structural information.In total, more than 1.5 million crystal structures have been refined and depositedin structural databanks (PDB, CSD and ICSD) to date. Almost 99.7% of themwere obtained by approximating atoms as spheres within the independent atommodel (IAM) introduced over a century ago. In this study, X-ray datasets forsingle crystals of hydrated-oxalic acid were refined using several alternativeelectron density models that abandon the crude spherical approximation: themultipole model (MM), the transferable aspherical atom model (TAAM) andthe Hirshfeld atom refinement (HAR) model as a function of the resolution ofX-ray data. The aspherical models (MM, TAAM, HAR) give far more accurateand precise single-crystal X-ray results than IAM, sometimes identical to resultsobtained from neutron diffraction and at low resolution. Hence, asphericalapproaches open new routes for improving existing structural informationcollected over the last century.