**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. |