Combined Twinning and Disorder

Combined Twinning and Disorder with SHELXL

Twinning and whole-molecule disorder can co-exist within the same crystal. This tutorial describes such a case: uric acid dihydrate. The dataset (*.hkl file) and a set of instructions (*.ins file) allow you to follow along. Although the combination of problems makes this structure more involved than previous examples, it is not difficult to get a stable refinement that makes complete chemical sense. The key to success is to proceed slowly, step-by-step. To give an idea of what you are aiming for, the final model should look something like this:

Unlike the whole-molecule disorder and twinning by pseudo-merohedry tutorials, we will start with structure solution, simply to prove that no magic is involved. The first step is to get an initial model for the structure. For uric acid dihydrate, the minor component fractions of both twinning and disorder can vary from crystal to crystal, which seems to be related to how the crystals are grown. In the structure presented here, the minor component fractions are fairly small compared to the major components. In such cases, minor components rarely prevent solving the structure. Here we'll use SHELXT for structure solution and ShelXle for model building.