# Locality Module¶

The locality module contains data structures to efficiently locate points based on their proximity to other points.

## NeighborList¶

class freud.locality.NeighborList

Class representing a certain number of “bonds” between particles. Computation methods will iterate over these bonds when searching for neighboring particles.

NeighborList objects are constructed for two sets of position arrays A (alternatively reference points; of length $$n_A$$) and B (alternatively target points; of length $$n_B$$) and hold a set of $$\left(i, j\right): i < n_A, j < n_B$$ index pairs corresponding to near-neighbor points in A and B, respectively.

For efficiency, all bonds for a particular reference particle i are contiguous and bonds are stored in order based on reference particle index i. The first bond index corresponding to a given particle can be found in $$log(n_{bonds})$$ time using find_first_index().

Module author: Matthew Spellings <mspells@umich.edu>

New in version 0.6.4.

Note

Typically, in python you will only manipulate a freud.locality.NeighborList object that you receive from a neighbor search algorithm, such as freud.locality.LinkCell and freud.locality.NearestNeighbors.

Example:

# assume we have position as Nx3 array
nlist = lc.nlist

# get all vectors from central particles to their neighbors
rijs = positions[nlist.index_j] - positions[nlist.index_i]
box.wrap(rijs)

copy(self, other=None)

Create a copy. If other is given, copy its contents into ourself. Otherwise, return a copy of ourself.

filter(self, filt)

Removes bonds that satisfy a boolean criterion.

Parameters: filt – Boolean-like array of bonds to keep (True => bond stays)

Note

This method modifies this object in-place

Example:

# keep only the bonds between particles of type A and type B
nlist.filter(types[nlist.index_i] != types[nlist.index_j])

filter_r(self, box, ref_points, points, float rmax, float rmin=0)

Removes bonds that are outside of a given radius range.

Parameters: ref_points – reference points to use for filtering points – target points to use for filtering rmax – maximum bond distance in the resulting neighbor list rmin – minimum bond distance in the resulting neighbor list

Note

This method modifies this object in-place

find_first_index(self, unsigned int i)

Returns the lowest bond index corresponding to a reference particle with index >=i

from_arrays(type cls, Nref, Ntarget, index_i, index_j, weights=None)

Create a NeighborList from a set of bond information arrays.

Parameters: Nref (unsigned int) – Number of reference points (corresponding to index_i) Ntarget (unsigned int) – Number of target points (corresponding to index_j) index_i (Array-like of unsigned ints, length num_bonds) – Array of integers corresponding to indices in the set of reference points index_j (Array-like of unsigned ints, length num_bonds) – Array of integers corresponding to indices in the set of target points weights (Array-like of floats, length num_bonds) – Array of per-bond weights (if None is given, use a value of 1 for each weight)
index_i

The reference point indices from the last set of points we were evaluated with. This array is read-only to prevent breakage of find_first_index.

index_j

The target point indices from the last set of points we were evaluated with. This array is read-only to prevent breakage of find_first_index.

neighbor_counts

A neighbor count array, which is an array of length N_ref indicating the number of neighbors for each reference particle from the last set of points we were evaluated with.

segments

A segment array, which is an array of length N_ref indicating the first bond index for each reference particle from the last set of points we were evaluated with.

weights

The per-bond weights from the last set of points we were evaluated with

class freud.locality.LinkCell(box, cell_width)

Supports efficiently finding all points in a set within a certain distance from a given point.

Module author: Joshua Anderson <joaander@umich.edu>

Parameters: box (freud.box.Box) – simulation box cell_width (float) – Maximum distance to find particles within

Note

freud.locality.LinkCell supports 2D boxes; in this case, make sure to set the z coordinate of all points to 0.

Example:

# assume we have position as Nx3 array
lc.computeCellList(box, positions)
for i in range(positions.shape[0]):
# cell containing particle i
cell = lc.getCell(positions[0])
# list of cell's neighboring cells
cellNeighbors = lc.getCellNeighbors(cell)
# iterate over neighboring cells (including our own)
for neighborCell in cellNeighbors:
# iterate over particles in each neighboring cell
for neighbor in lc.itercell(neighborCell):
pass # do something with neighbor index

# using NeighborList API
dens = density.LocalDensity(1.5, 1, 1)
dens.compute(box, positions, nlist=lc.nlist)

box

freud Box

compute(self, box, ref_points, points=None, exclude_ii=None)

Update the data structure for the given set of points and compute a NeighborList

Parameters: box (freud.box.Box) – simulation box ref_points (numpy.ndarray, shape= $$\left(N_{refpoints}, 3\right)$$, dtype= numpy.float32) – reference point coordinates points (numpy.ndarray, shape= $$\left(N_{points}, 3\right)$$, dtype= numpy.float32) – point coordinates exlude_ii – True if pairs of points with identical indices should be excluded; if None, is set to True if points is None or the same object as ref_points
computeCellList(self, box, ref_points, points=None, exclude_ii=None)

Update the data structure for the given set of points and compute a NeighborList

Parameters: box (freud.box.Box) – simulation box ref_points (numpy.ndarray, shape= $$\left(N_{refpoints}, 3\right)$$, dtype= numpy.float32) – reference point coordinates points (numpy.ndarray, shape= $$\left(N_{points}, 3\right)$$, dtype= numpy.float32) – point coordinates exlude_ii – True if pairs of points with identical indices should be excluded; if None, is set to True if points is None or the same object as ref_points
getBox(self)

Get the freud Box :return: freud Box :rtype: freud.box.Box

getCell(self, point)

Returns the index of the cell containing the given point

Parameters: point (numpy.ndarray, shape= $$\left(3\right)$$, dtype= numpy.float32) – point coordinates $$\left(x,y,z\right)$$ cell index unsigned int
getCellNeighbors(self, cell)

Returns the neighboring cell indices of the given cell

Parameters: cell (unsigned int) – Cell index array of cell neighbors numpy.ndarray, shape= $$\left(N_{neighbors}\right)$$, dtype= numpy.uint32
getNumCells(self)

Get the the number of cells in this box :return: the number of cells in this box :rtype: unsigned int

itercell(self, unsigned int cell)

Return an iterator over all particles in the given cell

Parameters: cell (unsigned int) – Cell index iterator to particle indices in specified cell iter
nlist

The neighbor list stored by this object, generated by compute.

num_cells

The number of cells in this box

## NearestNeighbors¶

class freud.locality.NearestNeighbors(rmax, n_neigh)

Supports efficiently finding the N nearest neighbors of each point in a set for some fixed integer N.

• strict_cut = True: rmax will be strictly obeyed, and any particle which
has fewer than N neighbors will have values of UINT_MAX assigned
• strict_cut = False: rmax will be expanded to find requested number of
neighbors. If rmax increases to the point that a cell list cannot be constructed, a warning will be raised and neighbors found will be returned

Module author: Eric Harper <harperic@umich.edu>

Parameters: rmax (float) – Initial guess of a distance to search within to find N neighbors n_neigh (unsigned int) – Number of neighbors to find for each point scale (float) – multiplier by which to automatically increase rmax value by if requested number of neighbors is not found. Only utilized if strict_cut is False. Scale must be greater than 1 strict_cut (bool) – whether to use a strict rmax or allow for automatic expansion

Example:

nn = NearestNeighbors(2, 6)
nn.compute(box, positions, positions)
hexatic = order.HexOrderParameter(2)
hexatic.compute(box, positions, nlist=nn.nlist)

UINTMAX

Value of C++ UINTMAX used to pad the arrays

box

freud Box

compute(self, box, ref_points, points, exclude_ii=None)

Update the data structure for the given set of points

Parameters: box (freud.box.Box) – simulation box ref_points (numpy.ndarray, shape=($$N_{particles}$$, 3), dtype= numpy.float32) – coordinated of reference points points (numpy.ndarray, shape=($$N_{particles}$$, 3), dtype= numpy.float32) – coordinates of points exlude_ii – True if pairs of points with identical indices should be excluded; if None, is set to True if points is None or the same object as ref_points
getBox(self)

Get the freud Box :return: freud Box :rtype: freud.box.Box

getNRef(self)

Get the the number of particles this object found neighbors of :return: the number of particles this object found neighbors of :rtype: unsigned int

getNeighborList(self)

Return the entire neighbors list

Returns: Neighbor List numpy.ndarray, shape= $$\left(N_{particles}, N_{neighbors}\right)$$, dtype= numpy.uint32
getNeighbors(self, unsigned int i)

Return the N nearest neighbors of the reference point with index i

Parameters: i (unsigned int) – index of the reference point to fetch the neighboring points of
getNumNeighbors(self)
Returns: the number of neighbors this object will find unsigned int
getRMax(self)

Return the current neighbor search distance guess :return: nearest neighbors search radius :rtype: float

getRsq(self, unsigned int i)

Return the Rsq values for the N nearest neighbors of the reference point with index i

Parameters: i (unsigned int) – index of the reference point of which to fetch the neighboring point distances squared distances of the N nearest neighbors Neighbor List numpy.ndarray, shape= $$\left(N_{particles}\right)$$, dtype= numpy.float32
getRsqList(self)

Return the entire Rsq values list

Returns: Rsq list numpy.ndarray, shape= $$\left(N_{particles}, N_{neighbors}\right)$$, dtype= numpy.float32
getUINTMAX(self)
Returns: value of C++ UINTMAX used to pad the arrays unsigned int
getWrappedVectors(self)

Return the wrapped vectors for computed neighbors. Array padded with -1 for empty neighbors

Returns: wrapped vectors numpy.ndarray, shape= $$\left(N_{particles}\right)$$, dtype= numpy.float32
n_ref

The number of particles this object found neighbors of

nlist

Returns the neighbor list stored by this object, generated by compute.

num_neighbors

The number of neighbors this object will find

r_max

Return the current neighbor search distance guess :return: nearest neighbors search radius :rtype: float

r_sq_list

Return the entire Rsq values list

Returns: Rsq list numpy.ndarray, shape= $$\left(N_{particles}, N_{neighbors}\right)$$, dtype= numpy.float32
setCutMode(self, strict_cut)

Set mode to handle rmax by Nearest Neighbors.

• strict_cut = True: rmax will be strictly obeyed, and any particle
which has fewer than N neighbors will have values of UINT_MAX assigned
• strict_cut = False: rmax will be expanded to find requested number of
neighbors. If rmax increases to the point that a cell list cannot be constructed, a warning will be raised and neighbors found will be returned
Parameters: strict_cut (bool) – whether to use a strict rmax or allow for automatic expansion
setRMax(self, float rmax)

Update the neighbor search distance guess :param rmax: nearest neighbors search radius :type rmax: float

wrapped_vectors

Return the wrapped vectors for computed neighbors. Array padded with -1 for empty neighbors