The use of effective core potentials in Hirshfeld atom refinement: making quantum crystallography faster in <i>NoSpherA2</i>

The refinement of structural models against X-ray diffraction data benefits significantly from employing non-spherical scattering factors in terms of precision and accuracy. Tailor-made scattering factors are available from quantum chemical calculations of the electron density using routine calculation packages. In this process, heavy elements in particular pose a difficulty due to their large numbers of electrons which are not involved in chemical bonding. An elegant way to circumvent this is by using effective core potentials. This work presents an approach for the treatment of these missing electrons in advanced structural refinement methods, such as Hirshfeld atom refinement. It also provides examples and benchmarks demonstrating up to a twofold reduction in refinement time without compromising on accuracy.