SOME DIFFERENT STRATEGIES OF LEAST-SQUARES REFINEMENT OF A MOLECULE

Refinements of a macromolecule (ribonuclease-A) based on structure amplitudes, \F\, and structure amplitude squares, \F\2, were carried out and the results compared. Although the conventional R values are higher for the \F\2 refinement, positional parameters from both types of refinement were not significantly different. However, the mean-square displacements from \F\2 refinements were systematically higher than for those using \F\. Various resolution windows and weighting schemes were employed during the work. Electron density maps were examined for \F\2 refinements and were very similar to those using \F\ in spite of a conventional R factor of 0.29 using all 1.4 angstrom data. While \F\2 refinements may be formally more correct than \F\ refinements, there is little evidence that \F\2 refinement is superior provided that a reasonable weighting strategy is adopted.