GAMGI Formats: Cell Create

# Cell Create

- Symmetry
- Volume
- Position
- Origin
- View

### system

system identifies the crystallographic system of the cell: cubic, tetragonal, orthorhombic, hexagonal, monoclinic or triclinic. Hexagonal, trigonal and rhombohedric cases are grouped in a single hexagonal system.

Although system is an optional parameter, system, lattice and group parameters together must provide enough information to properly identify the Bravais lattice of the cell. Redundant information is fine, but conflicting information is always flagged as an error.

    Example: <cell ... system="cubic"/> (no default)
    Allowed values: cubic, tetragonal, orthorhombic,
    hexagonal, monoclinic, triclinic (optional)

### lattice

lattice identifies the type of lattice centering of the cell: P, I, F, C or R. A, B and C centering is always described in GAMGI by C-based lattices. Thus monoclinic base-centered cells have always a unique axis b.

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).

Although lattice is an optional parameter, system, lattice and group parameters together must provide enough information to properly identify the Bravais lattice of the cell. Redundant information is fine, but conflicting information is always flagged as an error.

    Example: <cell ... lattice="P"/> (no default)
    Allowed values: P, I, F, C, R (optional)

### group

group identifies the space group of the cell: from 1 to 230. The four orthorhombic space groups 38-41, which are described as A groups when using the standard Hermann-Mauguin symbols, were converted to C groups 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.

Cells with an hexagonal system and a rombohedric R lattice (corresponding to the seven R space groups when using the standard Hermann-Mauguin symbols), are always represented using the hexagonal axes and the obverse setting, when the chosen volume is conventional, and the rombohedric axes, when the chosen volume is primitive. Cells with an hexagonal system and a primitive P lattice (corresponding to all the other space groups from 143 to 194 that are not R) are always represented using full hexagonal prismas, when the chosen volume is conventional, and one-third of the hexagonal prismas, when the chosen volume is primitive.

Although group is an optional parameter, system, lattice and group parameters together must provide enough information to properly identify the Bravais lattice of the cell. Redundant information is fine, but conflicting information is always flagged as an error.

    Example: <cell ... group="1"/> (no default)
    Allowed values: 1 - 230 (optional)

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

These lattice parameters define the cell geometry. 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.

    Example: <cell ... a="1.0" b="2.0" c="3.0"
    ab="50.0" bc="60.0" ac="70.0"/> (no default)
    Allowed values: positive real (required)

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