Lists and Tables

The esds in bond lengths, angles and torsion angles, chiral volumes, Ueq, and coefficients of least-squares planes and deviation of atoms from them, are estimated rigorously from the full correlation matrix (an approximate treatment is used for the angles between least-squares planes). The errors in the unit-cell dimensions (specified on the ZERR instruction) are taken into account exactly in estimating the esds in bond lengths, bond angles, torsion angles and chiral volumes. Correlation coefficients between the unit-cell dimensions are ignored except when determined by crystal symmetry (so that for a cubic crystal the cell esds contribute to errors in bond lengths and chiral volumes but not to the errors in bond angles or torsion angles). The (rather small) contributions of the unit-cell errors to the esds of quantities involving least-squares planes are estimated using an isotropic approximation.

For full-matrix refinement, the esds are calculated after the final refinement cycle. In the case of BLOC'ed refinement, the esds are calculated after every cycle (except that esds in geometric parameters are not calculated after pure Uij/sof cycles etc.), and the maximum estimate of each esd is printed. This prevents some esds being underestimated because not all of the relevant atoms were refined in the last cycle, but at the cost of overestimating all the esds if the R-factor drops appreciably during the refinement. Thus large structures should first be refined almost to convergence (either by CGLS or L.S./BLOC), and then a separate final blocked refinement job performed to obtain the final parameters and their esds. It is important that there is sufficient overlap between the blocks to enable every esd to be estimated with all contributing atoms refining in at least one of the refinement cycles.