Xtrapol8: automatic elucidation of low-occupancy intermediate-states in crystallographic studies

Unstable states studied in kinetic, time-resolved and ligand-based crystallography are often characterized by a low occupancy, hindering structure determination by conventional methods. To automatically extract such structures, we developed Xtrapol8, a program which (i) applies various flavors of Bayesian-statistics weighting to generate the most informative Fourier difference maps; (ii) determines the occupancy of the intermediate states by use of hitherto unavailable methods; (iii) calculates various types of extrapolated structure factors while handling the issue of negative structure factor amplitudes, and (iv) refines the corresponding structures.