GAMGI Interfaces: Cell Modify

# Cell Modify

- Symmetry
- Volume
- Position
- Origin
- View

Change here the symmetry and lattice parameters of the cell (or list of cells, previously selected with Cell->Select, when Global is pressed). Parameters for empty entries or Local choices remain unchanged.

To change the cell name write the new name in the Cell entry, followed by the cell number (GAMGI only needs the number to identify the cell).

### Symmetry

System, Lattice and Group are optional parameters, but together they must provide enough information to identify the Bravais lattice of the cell. Redundant information is fine, but conflicting information is flagged as an error.

When the entries System, Lattice or Group are changed, the other two are updated automatically. Predictable information is automatically written and conflicting information is automatically removed.

### System

System identifies the crystallographic system of the cell. The allowed values are: c (cubic), t (tetragonal), o (orthorhombic), h (hexagonal), m (monoclinic) and a (anorthic / triclinic). Hexagonal, trigonal and rhombohedric cases are grouped in GAMGI in a single hexagonal system, corresponding to the family designation used in the International Tables for Crystallography.

Pressing List, a second dialog shows all the systems that are compatible with the information currently inserted in the System, Lattice and Group entries.

### Lattice

Lattice identifies the lattice centering of the cell. The allowed values are: P (primitive), I (body-centered), C (face-centered), F (face-centered) and R (rhombohedral). A, B and C centering is always described in GAMGI by C-based lattices. Thus monoclinic centered cells are always C, with unique axis b and cell choice 1. To use data reported with a different cell choice or unique axis please use the transformation matrices published in International Tables for Crystallography.

The R lattices describe the seven space groups belonging to the hexagonal system that are described as R groups, when using the standard Hermann-Mauguin symbols: R3 (146), R-3 (148), R32 (155), R3m (160), R3c (161), R-3m (166), R-3c (167).

Pressing List, a second dialog shows all the lattices that are compatible with the information currently inserted in the System, Lattice and Group entries.

### Group

Group identifies the space group of the cell, using the numerical notation: 1 to 230.

The four orthorhombic space groups 38-41, which are described with A lattices when using the standard Hermann-Mauguin symbols, were converted to C lattices (so all base-centered lattices are described as C lattices) by using a different axes setting, which results from the axes permutation abc->bca, as described in the International Tables for Crystallography. These four groups, Amm2, Aem2, Ama2 and Aea2, are thus described in GAMGI as Cm2m, Cm2e, Cc2m and Cc2e, respectively.

For space groups 48, 50, 59, 68, 70 (orthorhombic), 85, 86, 88, 125, 126, 129, 130, 133, 134, 137, 138, 141, 142 (tetragonal), 201, 203, 222, 224, 227, 228 (cubic), there are two standard origins, denoted O1 or O2. In GAMGI the chosen origin is O2, at the inversion centre.

Unique axis b was used for all monoclinic space groups. For space groups 5, 7, 8, 9, 12, 13, 14, 15 (monoclinic), positions were constructed using Cell Choice 1.

Pressing List, a second dialog shows all the groups that are compatible with the information currently inserted in the System, Lattice and Group entries.

### a, b, c, ab, ac, bc

Entries a, b, c, ab, ac, bc permit to change the lengths and angles defining the conventional cell vectors (redundant entries are automatically disabled). Cell lengths must be positive, while cell angles must be larger than zero and smaller than 180 degrees.

For the cubic system, only one length is required (a or b or c). For the tetragonal system, two lengths are required (a or b and c). For the orthorhombic system, three lengths are required (a and b and c).

For the hexagonal system, two lengths are required (a or b and c). For the monoclinic system, three lengths are required (a and b and c), plus the angle around the axis b (ac).

For the triclinic system, all six parameters are required. Each angle must be smaller than the sum of the other two and must be larger than the absolute difference of the other two, otherwise an error is produced. Redundant information is fine, but conflicting information is always flagged as an error.
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