This section contains an example of how you can use the StructureData object to create complex crystals.

With the StructureData class we did not try to have a full set of features to manipulate crystal structures. Indeed, other libraries such as ASE exist, and we simply provide easy ways to convert between the ASE and the AiiDA formats. On the other hand, we tried to define a “standard” format for structures in AiiDA, that can be used across different codes.

# 3. How to use StructureData¶

Take a look at the following example:

alat = 4. # angstrom
cell = [[alat, 0., 0.,],
[0., alat, 0.,],
[0., 0., alat,],
]
s = StructureData(cell=cell)
s.append_atom(position=(0.,0.,0.), symbols='Fe')
s.append_atom(position=(alat/2.,alat/2.,alat/2.), symbols='O')


With the commands above, we have created a crystal structure s with a cubic unit cell and lattice parameter of 4 angstrom, and two atoms in the cell: one iron (Fe) atom in the origin, and one oxygen (O) at the center of the cube (this cell has been just chosen as an example and most probably does not exist).

Note

As you can see in the example above, both the cell coordinates and the atom coordinates are expressed in angstrom, and the position of the atoms are given in a global absolute reference frame.

In this way, any periodic structure can be defined. If you want to import from ASE in order to specify the coordinates, e.g., in terms of the crystal lattice vectors, see the guide on the conversion to/from ASE below.

When using the append_atom() method, further parameters can be passed. In particular, one can specify the mass of the atom, particularly important if you want e.g. to run a phonon calculation. If no mass is specified, the mass provided by NIST (retrieved in October 2014) is going to be used. The list of masses is stored in the module aiida.common.constants, in the elements dictionary.

Moreover, in the StructureData class of AiiDA we also support the storage of crystal structures with alloys, vacancies or partial occupancies. In this case, the argument of the parameter symbols should be a list of symbols, if you want to consider an alloy; moreover, you must pass a weights list, with the same length as symbols, and with values between 0. (no occupancy) and 1. (full occupancy), to specify the fractional occupancy of that site for each of the symbols specified in the symbols list. The sum of all occupancies must be lower or equal to one; if the sum is lower than one, it means that there is a given probability of having a vacancy at that specific site position.

As an example, you could use:

s.append_atom(position=(0.,0.,0.),symbols=['Ba','Ca'],weights=[0.9,0.1])


to add a site at the origin of a structure s consisting of an alloy of 90% of Barium and 10% of Calcium (again, just an example).

s.append_atom(position=(0.,0.,0.),symbols='Ca',weights=0.9)


would create a site with 90% probability of being occupied by Calcium, and 10% of being a vacancy.

Utility properties s.is_alloy and s.has_vacancies can be used to verify, respectively, if more than one element if given in the symbols list, and if the sum of all weights is smaller than one.

Note

if you pass more than one symbol, the property s.is_alloy will always be True, even if only one symbol has occupancy 1 and all others have occupancy zero:

>>> s = StructureData(cell=[[4,0,0],[0,4,0],[0,0,4]])
>>> s.append_atom(position=(0.,0.,0.), symbols=['Fe', 'O'], weights=[1.,0.])
>>> s.is_alloy
True


# 4. Internals: Kinds and Sites¶

Internally, the append_atom() method works by manipulating the kinds and sites of the current structure. Kinds are instances of the Kind class and represent a chemical species, with given properties (composing element or elements, occupancies, mass, …) and identified by a label (normally, simply the element chemical symbol).

Sites are instances of the Site class and represent instead each single site. Each site refers to a Kind to identify its properties (which element it is, the mass, …) and to its three spatial coordinates.

The append_atom() works in the following way:

• It creates a new Kind class with the properties passed as parameters (i.e., all parameters except position).

• It tries to identify if an identical Kind already exists in the list of kinds of the structure (e.g., in the same atom with the same mass was already previously added). Comparison of kinds is performed using aiida.orm.nodes.data.structure.Kind.compare_with(), and in particular it returns True if the mass and the list of symbols and of weights are identical (within a threshold). If an identical kind k is found, it simply adds a new site referencing to kind k and with the provided position. Otherwise, it appends k to the list of kinds of the current structure and then creates the site referencing to k. The name of the kind is chosen, by default, equal to the name of the chemical symbol (e.g., “Fe” for iron).

• If you pass more than one species for the same chemical symbol, but e.g. with different masses, a new kind is created and the name is obtained postponing an integer to the chemical symbol name. For instance, the following lines:

s.append_atom(position = [0,0,0], symbols='Fe', mass = 55.8)
s.append_atom(position = [1,1,1], symbols='Fe', mass = 57)
s.append_atom(position = [1,1,1], symbols='Fe', mass = 59)


will automatically create three kinds, all for iron, with names Fe, Fe1 and Fe2, and masses 55.8, 57. and 59. respecively.

• In case of alloys, the kind name is obtained concatenating all chemical symbols names (and a X is the sum of weights is less than one). The same rules as above are used to append a digit to the kind name, if needed.

• Finally, you can simply specify the kind_name to automatically generate a new kind with a specific name. This is the case if you want a name different from the automatically generated one, or for instance if you want to create two different species with the same properties (same mass, symbols, …). This is for instance the case in Quantum ESPRESSO in order to describe an antiferromagnetic cyrstal, with different magnetizations on the different atoms in the unit cell.

In this case, you can for instance use:

s.append_atom(position = [0,0,0], symbols='Fe', mass = 55.845, name='Fe1')
s.append_atom(position = [2,2,2], symbols='Fe', mass = 55.845, name='Fe2')


To create two species Fe1 and Fe2 for iron, with the same mass.

Note

You do not need to specify explicitly the mass if the default one is ok for you. However, when you pass explicitly a name and it coincides with the name of an existing species, all properties that you specify must be identical to the ones of the existing species, or the method will raise an exception.

Note

If you prefer to work with the internal Kind and Site classes, you can obtain the same result of the two lines above with:

from aiida.orm.nodes.data.structure import Kind, Site
s.append_kind(Kind(symbols='Fe', mass=55.845, name='Fe1'))
s.append_kind(Kind(symbols='Fe', mass=55.845, name='Fe1'))
s.append_site(Site(kind_name='Fe1', position=[0.,0.,0.]))
s.append_site(Site(kind_name='Fe2', position=[2.,2.,2.]))


# 5. Conversion to/from ASE¶

If you have an AiiDA structure s, you can get an ase.Atom object by just calling the aiida.orm.nodes.data.structure.StructureData.get_ase() method:

ase_atoms = s.get_ase()


Note

As we support alloys and vacancies in AiiDA, while ase.Atom does not, it is not possible to export to ASE a structure with vacancies or alloys.

If instead you have as ASE Atoms object and you want to load the structure from it, just pass it when initializing the class:

StructureData = DataFactory('structure')
# or:
# from aiida.orm import StructureData
aiida_structure = StructureData(ase = ase_atoms)


## 5.1. Creating multiple species¶

We implemented the possibility of specifying different Kinds (species) in the ase.atoms and then importing them.

In particular, if you specify atoms with different mass in ASE, during the import phase different kinds will be created:

>>> import ase
>>> StructureData = DataFactory("structure")
>>> ase_structure = ase.Atoms('Fe2')
>>> ase_structure[0].mass = 55.
>>> ase_structure[1].mass = 56.
>>> ase_structure.cell = cell # defines a periodic cell
>>> s = StructureData(ase=ase_structure)
>>> for kind in s.kinds:
>>>     print(kind.name, kind.mass)
Fe 55.0
Fe1 56.0


Moreover, even if the mass is the same, but you want to get different species, you can use the ASE tags to specify the number to append to the element symbol in order to get the species name:

>>> import ase
>>> StructureData = DataFactory("structure")
>>> ase_structure = ase.Atoms('Fe2')
>>> ase_structure[0].tag = 1
>>> ase_structure[1].tag = 2
>>> ase_structure.cell = cell
>>> s = StructureData(ase=ase_structure)
>>> for kind in s.kinds:
>>>     print(kind.name)
Fe1
Fe2


Note

in complicated cases (multiple tags, masses, …), it is possible that exporting a AiiDA structure to ASE and then importing it again will not perfectly preserve the kinds and kind names.

# 6. Conversion to/from pymatgen¶

AiiDA structure can be converted to pymatgen’s Molecule and Structure objects by using, accordingly, get_pymatgen_molecule() and get_pymatgen_structure() methods:

pymatgen_molecule  = aiida_structure.get_pymatgen_molecule()
pymatgen_structure = aiida_structure.get_pymatgen_structure()


A single method get_pymatgen() can be used for both tasks: converting periodic structures (periodic boundary conditions are met in all three directions) to pymatgen’s Structure and other structures to pymatgen’s Molecule:

pymatgen_object = aiida_structure.get_pymatgen()


It is also possible to convert pymatgen’s Molecule and Structure objects to AiiDA structures:

StructureData = DataFactory("structure")
from_mol      = StructureData(pymatgen_molecule=mol)
from_struct   = StructureData(pymatgen_structure=struct)


Also in this case, a generic converter is provided:

StructureData = DataFactory("structure")
from_mol      = StructureData(pymatgen=mol)
from_struct   = StructureData(pymatgen=struct)


Note

Converters work with version 3.0.13 or later of pymatgen. Earlier versions may cause errors.