Events

25 May

Crystal structure determination by diffractometric direct methods: Evolution and recent advances

Monday 25 May 2020, 12:00pm

Online ICMAB Periodical Lecture - REGISTER HERE

by Jordi Rius, Crystallography Group, ICMAB-CSIC

Abstract: Since their beginning in the middle of the past century, Direct Methods (DM) have played a central role in the determination of crystal structures. Due to the increasing computing power, structure solution strategies and computing algorithms have continuously adapted to take full advantage of this change. The principal stages in the evolution of (automated) DM will be illustrated with the help of Patterson-function direct methods (PFDM) [1].

This especial sort of DM solves the unknown crystal structures by direct interpretation of the corresponding origin-free modulus functions in terms of the structure factor phases (F). The optimization of the F phases is achieved by maximizing the so-called SM modulus sum function through an improved algorithm actively using strong and weak intensities without needing cumbersome weighting schemes [2].

This algorithm has been implemented in the XLENS software [3] and can work either in reciprocal space (SM-TF) or also alternating reciprocal and real spaces (SM-FFT) [4]. Recently, the latter SM-FFT algorithm has been improved by exploiting the similarity between the r2 and |r| density functions which allows handling larger structures (up to 400 atoms in the asymmetrical unit) more efficiently [5].

Some examples on the application of PFDM to the solution of inorganic and organic crystal structures from powder diffraction data [6], from 3D electron diffraction data [7] and from synchrotron through-the-substrate microdiffraction data [8] (extracted with the D2dplot software [9]) will be given.

[1] J. Rius, IUCrJ 1 (2014) 291. [2] J. Rius, Acta Cryst. A49 (1993) 506. [3] J. Rius, XLENS® software: Available at departments.icmab.es/crystallography/software. [4] J. Rius, A. Crespi, X.Torrelles, Acta Cryst. A63 (2007) 131. [5] J. Rius, Acta Cryst A (accepted). [6] J. Rius, Acta Cryst. A67 (2011) 63. [7] J. Rius, E. Mugnaioli, O. Vallcorba, U. Kolb, Acta Cryst. A69 (2013) 396. [8] J. Rius et al., IUCrJ 2 (2015) 452. [9] O. Vallcorba, J. Rius, J. Appl.Cryst. 52 (2019) 478. 

 

Hosted by Carles Miravitlles, former ICMAB Director, and Xavier Obradors, current ICMAB Director