This tag should be applied to questions that deal with the particular arrangement of atoms in a molecule, the so-called molecular structure or geometry.
The molecular structure, geometry, or the structure of a molecule is the particular relative arrangement of atoms in a molecule. The term geometry is widely used in theoretical-chemistry, computational-chemistry and related branches.
A very common format for sharing geometries are cartesian coordinates (in Ångstrom). For example, in ammonia, $\ce{NH3}$ the structure is well defined by the following code (including the calculated electronic energy at the BP86/cc-pVDZ level of theory).
4
AMMONIA, BP86/cc-pVDZ: -56.553652 E(H)
N 0.000000 0.000000 0.000000
H 0.000000 0.000000 1.034364
H 1.006493 0.000000 -0.238496
H -0.300271 -0.960659 -0.238496
A geometry can also be shared in different coordinates. The following is a z-Matrix in the Gaussian09 file format. It makes use of the internal symmetry of the molecule.
n
h 1 hn2
h 1 hn2 2 hnh3
h 1 hn2 2 hnh3 3 dih4
hn2 1.0344
hnh3 103.3308
dih4 107.3576
Most commonly the geometry is displayed in a picture:
