Composite model refinement

Definition

Composite model refinement is a technique used in crystallography to improve the accuracy of a crystal structure by combining multiple models that represent different conformations or arrangements of the atoms within a single crystal. This approach is particularly valuable when dealing with complex structures that may exhibit disorder or twinning, allowing researchers to account for variations in the atomic positions and enhance the quality of the structural data.

5 Must Know Facts For Your Next Test

1. Composite model refinement can significantly improve R-factors by incorporating different conformational states into the model, resulting in a better fit to experimental data.
2. This technique is particularly useful in systems where static models fail to adequately represent the flexibility or variability observed in experimental electron density maps.
3. By employing composite models, researchers can distinguish between different functional forms or conformations of molecules within a single crystal lattice.
4. Careful consideration must be given to the selection and weighting of different models during composite refinement to ensure meaningful results.
5. Advanced software tools are often required to effectively implement composite model refinement, as the complexity of multiple conformers increases computational demands.

Review Questions

• How does composite model refinement enhance the accuracy of crystal structure determination?
  • Composite model refinement enhances accuracy by integrating multiple conformational states into a single model, which helps to account for variations in atomic positions that standard single-model refinements might overlook. This technique improves the fit between observed and calculated diffraction data, leading to more reliable structural insights. By addressing complexities like disorder and flexibility in molecules, composite refinements provide a clearer representation of the actual molecular arrangement in crystals.
• What role does disorder play in influencing the need for composite model refinement in crystallography?
  • Disorder introduces uncertainty in atomic positions, making it challenging to accurately define a crystal structure using traditional methods. When molecules exhibit multiple possible configurations due to thermal vibrations or dynamic movements, standard refinements can lead to misleading results. Composite model refinement addresses this issue by allowing for multiple positions of atoms to be included in the model, improving the overall representation of disorder and enhancing structural accuracy.
• Evaluate the implications of using composite model refinement on interpreting complex crystal structures involving twinning.
  • Using composite model refinement on twinned crystal structures can significantly enhance our understanding of molecular interactions and dynamics. Twinning complicates data analysis by introducing overlapping diffraction patterns from different domains, making it difficult to extract accurate structural information. By employing composite models that account for these complexities, researchers can effectively separate contributions from different domains and refine their understanding of molecular behavior. This leads to richer insights into material properties and functions, showcasing the importance of advanced refinement techniques in modern crystallography.

Related terms

Twinning: A phenomenon where two or more crystal domains share some of the same lattice points, leading to complex diffraction patterns that can complicate data analysis.

Disorder: The presence of multiple possible positions for atoms within a crystal structure, which can arise from thermal vibrations or dynamic movements, complicating the interpretation of electron density maps.

R-factor: A numerical value that indicates the quality of a crystallographic model; lower R-factors suggest better agreement between observed and calculated data.