BYPASS: an effective method for the refinement of crystal structures containing disordered solvent regions

TLDR

The paper presents a method for least‑squares refinement of ordered atomic parameters in crystal structures with disordered solvent regions. The method automatically identifies solvent regions, calculates their Fourier‑transform contributions, incorporates them into iterative least‑squares refinement of the ordered part, and is applied to four crystal structures. The approach markedly improves atomic geometry and uncertainties, and the final difference map allows estimation of solvent molecule count in the unit cell.

Abstract

A method is described for the least-squares refinement of the atomic parameters of the ordered part of a crystal structure in the presence of disordered solvent areas. Potential solvent regions are identified automatically. The contribution of the observed contents to the total structure factor is calculated via a discrete Fourier transformation, and incorporated in a further least-squares refinement of the ordered part of the structure. The procedure is iterated a few times to convergence. It is found that this mixed discrete-atom and continuous solvent-area model refinement approach greatly improves the quality of discrete atomic parameters, i.e. the geometry and the e.s.d.'s. An electron count over the solvent region in the final difference electron-density map provides a convenient estimate for the number of solvent molecules present in the unit cell. The application of the method to four structures is described.