1Both theoretical databases show essentially the same performance in the structure refinement of the undecapeptide cyclosporin, the small-molecule pharmaceutical paracetamol and the dipeptide L-histidinyl-L-alanine dihydrate.
2Modeling H atoms in experimental charge density studies is usually limited to lmax = 2 rather than lmax = 4 for other elements. This is due to the small scattering contribution of H atoms, which is limited in resolution, and due to convolution of electron density with thermal motion.
3Torsion angles involving next-nearest neighbors only need to agree for delocalized bonding environments, but can differ without compromising transferability for single-bond environments.
4Localization can be visualized using the electron localization function, ELF (Becke & Edgebombe, 1990), the more evolved electron localization indicator, ELI (Wagner et al., 2007), or the source function (Gatti, 2005).
5Terminating valences with H atoms leads to smaller model compounds and hence also minimizes the computational effort of quantum chemical calculations.
6In case the atom of interest or its neighbors are involved in a condensed ring system this rule might lead to relatively large model compounds. More importantly it fulfills the requirement to include `the whole mesomeric system' (Luger & Dittrich, 2007).
7Note that for disordered crystal structures where split atoms are in close proximity, the analysis of geometry leads to meaningless invariom names.
8Supplementary data for this paper are available from the IUCr electronic archives (Reference: SN5118 ). Services for accessing these data are described at the back of the journal.
9For C atoms only but no ' parameter seems to be required.
10We chose to use the term `simulated' because we simulate an X-ray diffraction experiment. Furthermore, the term `calculated structure factors' is already being used and well established for model structure factors, and using it would cause confusion.
11Taking planarity into account was inspired by the procedure used in the UBDB (Dominiak et al., 2007).
12We have discussed e.g. using CIP rules to elegantly cover tetrahedral C atoms, but these would unfortunately not cover other bonding environments.