International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

International Tables for Crystallography (2006). Vol. F, ch. 25.2, p. 713   | 1 | 2 |

## Figure 25.2.3.4

A. T. Brunger,v* P. D. Adams,e W. L. DeLano,f P. Gros,g R. W. Grosse-Kunstleve,e J.-S. Jiang,h N. S. Pannu,i R. J. Read,j L. M. Ricek and T. Simonsonl
 Figure 25.2.3.4 Examples of symbolic definition of a refinement target function and its derivatives with respect to the calculated structure-factor arrays. (a) Simultaneous refinement of heavy-atom sites of three derivatives. The target function is defined by the target' expression. ', ' and ' (in bold) are complex structure factors corresponding to three sets of heavy atoms that are specified using atom selections [equation (25.2.3.7)]. The target function and its derivatives with respect to the three structure-factor arrays are defined symbolically using the structure-factor amplitudes of the native crystal, ', those of the derivatives, ', ', ', the complex structure factors of the heavy-atom models, ', ', ', and the corresponding lack-of-closure variances, ', ' and '. The summation over the selected stucture factors (tselection') is performed implicitly. (b) Refinement of two independent models against perfectly twinned data. fcalc1' and `fcalc2' are complex structure factors for the models that are related by a twinning operation (in bold). The target function and its derivatives with respect to the two structure-factor arrays are explicitly defined.