Cell Define Window

Figure 5.3. Cell Definition Window

Allows the user to edit the unit cell specification of the current model, assign a crystallographic spacegroup, and perform spacegroup packing according to the spacegroup.

Whether or not the current model possesses a unit cell is determined wholly by the 'Has Cell' checkbox - if checked, the current model is periodic, if not, then it is an isolated atomic globule (or a 'molecule', if you prefer). If the model has a cell, its lengths (in Angstroms) and angles (in degrees) are displayed on the left-hand side, and may be freely edited (the volume of the current cell is displayed just below). Any changes made are immediately applied to the model. Any changes made here are immediately applied to the current model.

The spacegroup assigned to the current model can be changed and removed here. On its own, an assigned spacegroup is just a number - no modifications to the atoms are made by changing the assigned spacegroup. Packing of atoms, however, is a different matter. Once a spacegroup is assigned, symmetry-equivalent atoms may be generated by use of the 'Pack' button. This assumes that the current contents of the model represents the symmetry-unique set of atoms (i.e. the minimal generator set which, along with the spacegroup and unit cell, defines the entire crystal).