Define here the symmetry and lattice parameters of 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.
### 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.
### 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.
### Group

**Group** identifies the space group of the cell, using the numerical
notation: **1** to **230**.
### a, b, c, ab, ac, bc

Entries **a**, **b**, **c**, **ab**, **ac**, **bc**
permit to set 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.

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.

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

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.

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.

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.