Code dependence of calculated crystalline electron densities. Possible lessons for quantum crystallography.

The use of electronic structure methods in crystallographic data analysis, the now well known field of quantum crystallography, aids in the solution of several problems in X-ray diffraction refinement, as well as opening new avenues to access a whole new set of experimentally available observables. A key ingredient in quantum crystallography is the theoretically derived electron density, ρ, obtained from standard electronic structure codes. Here, we introduce a factor that has not been carefully considered until now. As we demonstrate, theoretically derived ρ values depend not only on the set of computational conditions used to obtain them but also on the particular computational code selected for this task. We recommend that all quantum crystallographers carefully check the convergence of ρ before undertaking any serious study.