Cluster space

Setting up a cluster space

The cluster space serves as a basis for cluster expansions. Setting up a clusterspace object is commonly done via

>>> from icet import ClusterSpace
>>> from ase.build import bulk
>>> primitive_structure = bulk('Si')
>>> cs = ClusterSpace(primitive_structure, cutoffs=[7.0, 5.0], chemical_symbols=['Si', 'Ge'])

The cutoffs are set up to include pairs with an interatomic distance smaller than 7 Å and triplets for which all pair-wise interatomic distances are smaller than 5 Å. Here, Si and Ge are allowed to occupy the reference lattice.

Sublattices

A cluster space can also be constructed with multiple sublattices. Consider for example a rocksalt lattice, on which we want to allow mixing of Na and Li on the Na sublattice and Cl and F on the Cl sublattices. This can be achived by

>>> from ase.build import bulk
>>> atoms = bulk('NaCl', 'rocksalt', a=4.0)
>>> chemical_symbols = [['Na', 'Li'], ['Cl', 'F']]
>>> cs = ClusterSpace(atoms, [7.0, 5.0], chemical_symbols)

where chemical_symbols now specifies which species are allowed for each lattice site in atoms.

Inactive sites

The sublattice functionality also allows one to have inactive sites. For example, if we consider the system above but would like to keep the Cl lattice fixed it can be achived via

>>> chemical_symbols = [['Na', 'Li'], ['Cl']]
>>> cs = ClusterSpace(atoms, [7.0, 5.0], chemical_symbols)

2D systems

icet requires input structures to have periodic boundary conditions (PBCs). In order to treat two-dimensional systems, or more generally without PBCs in at least one direction, one has to surround the prototype structure with vacuum and then apply PBCs in all directions. This can easily be achived with the ASE functions

>>> from ase.build import surface
>>> atoms = surface('Cu', (1,1,1), layers=7)

which creates ase atoms object without PBC in the z-direction. The structure can be modified to have PBC in the z-direction with vacuum via

>>> atoms.pbc = [True, True, True]
>>> atoms.center(vacuum=20, axis=2)

which can now be used to create a cluster space.

Interface

class icet.ClusterSpace(structure, cutoffs, chemical_symbols, symprec=1e-05, position_tolerance=None)

This class provides functionality for generating and maintaining cluster spaces.

Note

In icet all Atoms objects must have periodic boundary conditions. When constructing cluster expansions for surfaces and nanoparticles it is therefore recommended to surround the structure with vacuum and use periodic boundary conditions. This can be achieved by using Atoms.center.

Parameters:

• structure (Atoms) – Atomic structure.

• cutoffs (List[float]) –

  Cutoff radii per order that define the cluster space.

  Cutoffs are specified in units of Ångstrom and refer to the longest distance between two atoms in the cluster. The first element refers to pairs, the second to triplets, the third to quadruplets, and so on. cutoffs=[7.0, 4.5] thus implies that all pairs distanced 7 Å or less will be included, as well as all triplets among which the longest distance is no longer than 4.5 Å.

• chemical_symbols (Union[List[str], List[List[str]]]) –

  List of chemical symbols, each of which must map to an element of the periodic table.

  If a list of chemical symbols is provided, all sites on the lattice will have the same allowed occupations as the input list.

  If a list of list of chemical symbols is provided then the outer list must be the same length as the structure object and chemical_symbols[i] will correspond to the allowed species on lattice site i.

• symprec (float) – Tolerance imposed when analyzing the symmetry using spglib.

• position_tolerance (Optional[float]) – Tolerance applied when comparing positions in Cartesian coordinates.

Examples

The following snippets illustrate several common situations:

>>> from ase.build import bulk
>>> from ase.io import read
>>> from icet import ClusterSpace

>>> # AgPd alloy with pairs up to 7.0 A and triplets up to 4.5 A
>>> prim = bulk('Ag')
>>> cs = ClusterSpace(structure=prim, cutoffs=[7.0, 4.5],
...                   chemical_symbols=[['Ag', 'Pd']])
>>> print(cs)

>>> # (Mg,Zn)O alloy on rocksalt lattice with pairs up to 8.0 A
>>> prim = bulk('MgO', crystalstructure='rocksalt', a=6.0)
>>> cs = ClusterSpace(structure=prim, cutoffs=[8.0],
...                   chemical_symbols=[['Mg', 'Zn'], ['O']])
>>> print(cs)

>>> # (Ga,Al)(As,Sb) alloy with pairs, triplets, and quadruplets
>>> prim = bulk('GaAs', crystalstructure='zincblende', a=6.5)
>>> cs = ClusterSpace(structure=prim, cutoffs=[7.0, 6.0, 5.0],
...                   chemical_symbols=[['Ga', 'Al'], ['As', 'Sb']])
>>> print(cs)

>>> # PdCuAu alloy with pairs and triplets
>>> prim = bulk('Pd')
>>> cs = ClusterSpace(structure=prim, cutoffs=[7.0, 5.0],
...                   chemical_symbols=[['Au', 'Cu', 'Pd']])
>>> print(cs)

property as_list: List[dict]

Representation of cluster space as list with information regarding order, radius, multiplicity etc.

assert_structure_compatibility(structure, vol_tol=1e-05)

Raises error if structure is not compatible with this cluster space.

Parameters:

• structure (Atoms) – Structure to check for compatibility with cluster space.

• vol_tol (float) – Tolerance imposed when comparing volumes.

Return type:

None

property chemical_symbols: List[List[str]]

Species identified by their chemical symbols.

copy()

Returns copy of ClusterSpace instance.

property cutoffs: List[float]

Cutoffs for different n-body clusters. The cutoff radius (in Ångstroms) defines the largest interatomic distance in a cluster.

property fractional_position_tolerance: float

Tolerance applied when comparing positions in fractional coordinates.

get_cluster_vector(structure)

Returns the cluster vector for a structure.

Parameters:

structure (Atoms) – Atomic configuration.

Return type:

ndarray

get_coordinates_of_representative_cluster(orbit_index)

Returns the positions of the sites in the representative cluster of the selected orbit.

Parameters:

orbit_index (int) – Index of the orbit for which to return the positions of the sites.

Return type:

List[Tuple[float]]

get_possible_orbit_occupations(orbit_index)

Returns possible occupations of the orbit.

Parameters:

orbit_index (int) – Index of orbit of interest.

Return type:

List[List[str]]

get_sublattices(structure)

Returns the sublattices of the input structure.

Parameters:

structure (Atoms) – Atomic structure the sublattices are based on.

Return type:

Sublattices

is_supercell_self_interacting(structure)

Checks whether a structure has self-interactions via periodic boundary conditions. Returns True if the structure contains self-interactions via periodic boundary conditions, otherwise False.

Parameters:

structure (Atoms) – Structure to be tested.

Return type:

bool

merge_orbits(equivalent_orbits, ignore_permutations=False)

Combines several orbits into one. This allows one to make custom cluster spaces by manually declaring the clusters in two or more orbits to be equivalent. This is a powerful approach for simplifying the cluster spaces of low-dimensional structures such as surfaces or nanoparticles.

The procedure works in principle for any number of components. Note, however, that in the case of more than two components the outcome of the merging procedure inherits the treatment of the multi-component vectors of the orbit chosen as the representative one.

Parameters:

• equivalent_orbits (Dict[int, List[int]]) – The keys of this dictionary denote the indices of the orbit into which to merge. The values are the indices of the orbits that are supposed to be merged into the orbit denoted by the key.

• ignore_permutations (bool) – If True orbits will be merged even if their multi-component vectors and/or site permutations differ. While the object will still be functional, the cluster space may not be properly spanned by the resulting cluster vectors.

Return type:

None

Note

The orbit index should not be confused with the index shown when printing the cluster space.

Examples

The following snippet illustrates the use of this method to create a cluster space for a (111) FCC surface, in which only the singlets for the first and second layer are distinct as well as the in-plane pair interaction in the topmost layer. All other singlets and pairs are respectively merged into one orbit. After merging there aree only 3 singlets and 2 pairs left with correspondingly higher multiplicities.

>>> from icet import ClusterSpace
>>> from ase.build import fcc111
>>>
>>> # Create primitive surface unit cell
>>> structure = fcc111('Au', size=(1, 1, 8), a=4.1, vacuum=10, periodic=True)
>>>
>>> # Set up initial cluster space
>>> cs = ClusterSpace(structure=structure, cutoffs=[3.8], chemical_symbols=['Au', 'Ag'])
>>>
>>> # At this point, one can inspect the orbits in the cluster space by printing the
>>> # ClusterSpace object and accessing the individial orbits.
>>> # There will be 4 singlets and 8 pairs.
>>>
>>> # Merge singlets for the third and fourth layers as well as all pairs except for
>>> # the one corresponding to the in-plane interaction in the topmost surface
>>> # layer.
>>> cs.merge_orbits({2: [3], 4: [6, 7, 8, 9, 10, 11]})

property number_of_orbits_by_order: dict

Number of orbits by order in the form of a dictionary where keys and values represent order and number of orbits, respectively.

property orbit_list

Orbit list that defines the cluster in the cluster space.

property position_tolerance: float

Tolerance applied when comparing positions in Cartesian coordinates.

property primitive_structure: Atoms

Primitive structure on which cluster space is based.

static read(filename)

Reads cluster space from file and returns ClusterSpace object.

Parameters:

filename (str) – Name of file from which to read cluster space.

property space_group: str

Space group of the primitive structure in international notion (via spglib).

property symprec: float

Tolerance imposed when analyzing the symmetry using spglib.

to_dataframe()

Returns a representation of the cluster space as a DataFrame.

Return type:

DataFrame

write(filename)

Saves cluster space to a file.

Parameters:

filename (str) – Name of file to which to write.

Return type:

None