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)