title: Crystal structures of increasingly large molecules: meeting the challenges with CRYSTALS software.
abstract: BACKGROUND
  The size and complexity of molecules being studied by single crystal diffraction is growing year by year, resulting in an increase in the difficulties encountered during structure determination. From the crystallisation itself and sample handling, to structure solution and refinement, specific problems due to larger molecules are discussed.
  
  
  RESULTS
  During refinement, several methods are available to deal with the problems encountered with large structures within the software Crystals. Hydrogens atoms can neither be found easily nor refined freely, but restraints can be applied automatically. Special scattering factors can be used to model complex disorder. Finally chemical information can be included in the form of restraints in order to help the determination of a good model. Multicollinearity problems are more likely in the refinement of large structures; to some extent more precise and accurate algorithms can help. Also, if the global minimum is less well defined, faster refinement enables more cycles to be carried out, a necessity for good convergence. The efficiency of the algorithms in Crystals have been increased to help address these issues.
  
  
  CONCLUSIONS
  Thus, crystal structures are getting larger and their complexity is increasing. Recent developments in precision and speed during the least squares in Crystals is helping the structural scientist to deal with larger structures more efficiently.
authors.affiliation: Chemical Crystallography, Chemistry Research Laboratory, Mansfield Road, Oxford, OX1 3TA UK.
authors.forename: Pascal
authors.initials: P
authors.lastname: Parois
authors.affiliation: Chemical Crystallography, Chemistry Research Laboratory, Mansfield Road, Oxford, OX1 3TA UK.
authors.forename: Richard I
authors.initials: RI
authors.lastname: Cooper
authors.affiliation: Chemical Crystallography, Chemistry Research Laboratory, Mansfield Road, Oxford, OX1 3TA UK.
authors.forename: Amber L
authors.initials: AL
authors.lastname: Thompson
keywords: Crystals software; Large molecule crystal structures; Least-squares refinement