Recovering experimental and theoretical electron densities in corundum using the multipolar model: IUCr Multipole Refinement Project

This electron-density study on corundum (alpha -Al2O3) is part of the Multi pole Refinement Project supported by the IUCr Commission on Charge, Spin an d Momentum Densities. For this purpose, eight different data sets (two expe rimental and six theoretical) were chosen from which the electron density w as derived by multipolar refinement (using the MOLLY program). The two expe rimental data sets were collected on a conventional CAD4 and at ESRF, ID11 with a CCD detector, respectively. The theoretical data sets consist of sta tic, dynamic, static noisy and dynamic noisy moduli of structure factors ca lculated at the Hartree-Fock (HF) and density functional theory (DFT) level s. Comparisons of deformation and residual densities show that the multipol ar analysis works satisfactorily but also indicate some drawbacks in the re finement. Some solutions and improvements during the refinements are propos ed like contraction or expansion of the inner atomic shells or increasing t he order of the spherical harmonic expansion.