Advances in the free lunch method

The most critical limit of macromolecular crystallography, the experimental data resolution, is partially `tricked' by the `free lunch method' (non-measured reflection extrapolation). The best electron density map available when only observed data are used may be employed to extrapolate moduli and phases of unobserved reflections behind and beyond the experimental resolution limit. The method is able to reduce the mean phase error of the observed reflections and to produce a more interpretable (in terms of a molecular model) electron density map. The main features of the free lunch method have been studied and its performance has been enhanced; it is beneficial even if data resolution is about 2 Å. Furthermore, the technique has been parameterized so that it may be routinely used by other phasing programs.