# Geometry creation – part 1¶

To create a Geometry one needs to define a set of attributes. The only required information is the atomic coordinates:

>>> single_hydrogen = Geometry([[0., 0., 0.]])
>>> print(single_hydrogen)
{na: 1, no: 1, species:
{Atoms(1):
(1) == [H, Z: 1, orbs: 1, mass(au): 1.00794, maxR: -1.00000],
},
nsc: [1, 1, 1], maxR: -1.0
}


this will create a Geometry object with 1 Hydrogen atom with a single orbital (default if not specified), and a supercell of 10 A in each Cartesian direction. When printing a Geometry object a list of information is printed in an XML-like fashion. na corresponds to the total number of atoms in the geometry, while no refers to the total number of orbitals. The species are printed in a sub-tree and Atoms(1) means that there is one distinct atomic specie in the geometry. That atom is a Hydrogen, with mass listed in atomic-units. maxR refers to the maximum range of all the orbitals associated with that atom. A negative number means that there is no specified range. Lastly nsc refers to the number of neighbouring super-cells that is represented by the object. In this case [1, 1, 1] means that it is a molecule and there are no super-cells (only the unit-cell).

To specify the atomic specie one may do:

>>> single_carbon = Geometry([[0., 0., 0.]], Atom('C'))


which changes the Hydrogen to a Carbon atom. See <link to atom_01.rst> on how to create different atoms.

To create a geometry with two different atomic species, for instance a chain of alternating Natrium an Chloride atoms, separated by 1.6 A one may do:

>>> chain = Geometry([[0. , 0., 0.],
[1.6, 0., 0.]], [Atom('Na'), Atom('Cl')],
[3.2, 10., 10.])


note the last argument which specifies the Cartesian lattice vectors. sisl is clever enough to repeat atomic species if the number of atomic coordinates is a multiple of the number of passed atoms, i.e.:

>>> chainx2 = Geometry([[0. , 0., 0.],
[1.6, 0., 0.],
[3.2, 0., 0.],
[4.8, 0., 0.]]], [Atom('Na'), Atom('Cl')],
[6.4, 10., 10.])


which is twice the length of the first chain with alternating Natrium and Chloride atoms, but otherwise identical.

This is the most basic form of creating geometries in sisl and is the starting point of almost anything related to sisl.