# Irreps

A group representation $$(D,V)$$ describe the action of a group $$G$$ on a vector space $$V$$

$D : G \longrightarrow \text{linear map on } V.$

The irreducible representations, in short irreps (definition of irreps) are the “smallest” representations.

• Any representation can be decomposed via a change of basis into a direct sum of irreps

• Any physical quantity, under the action of $$O(3)$$, transforms with a representation of $$O(3)$$

The irreps of $$SO(3)$$ are called the wigner matrices $$D^L$$. The irreps of the group of inversion ($$\{e, I\}$$) are the trivial representation $$\sigma_+$$ and the sign representation $$\sigma_-$$

$\begin{split}\sigma_p(g) = \left \{ \begin{array}{l} 1 \text{ if } g = e \\ p \text{ if } g = I \end{array} \right..\end{split}$

The group $$O(3)$$ is the direct product of $$SO(3)$$ and inversion

$g = r i, \quad r \in SO(3), i \in \text{inversion}.$

The irreps of $$O(3)$$ are the product of the irreps of $$SO(3)$$ and inversion. An instance of the class e3nn.o3.Irreps represent a direct sum of irreps of $$O(3)$$:

$g = r i \mapsto \bigoplus_{j=1}^n m_j \times \sigma_{p_j}(i) D^{L_j}(r)$

where $$(m_j \in \mathbb{N}, p_j = \pm 1, L_j = 0,1,2,3,\dots)_{j=1}^n$$ defines the e3nn.o3.Irreps.

Irreps of $$O(3)$$ are often confused with the spherical harmonics, the relation between the irreps and the spherical harmonics is explained at Spherical Harmonics.

class e3nn.o3.Irrep(l: , p=None)[source]

Bases: tuple

Irreducible representation of $$O(3)$$

This class does not contain any data, it is a structure that describe the representation. It is typically used as argument of other classes of the library to define the input and output representations of functions.

Parameters
• l (int) – non-negative integer, the degree of the representation, $$l = 0, 1, \dots$$

• p ({1, -1}) – the parity of the representation

Examples

Create a scalar representation ($$l=0$$) of even parity.

>>> Irrep(0, 1)
0e


Create a pseudotensor representation ($$l=2$$) of odd parity.

>>> Irrep(2, -1)
2o


Create a vector representation ($$l=1$$) of the parity of the spherical harmonics ($$-1^l$$ gives odd parity).

>>> Irrep("1y")
1o

>>> Irrep("2o").dim
5

>>> Irrep("2e") in Irrep("1o") * Irrep("1o")
True

>>> Irrep("1o") + Irrep("2o")
1x1o+1x2o


Methods:

 D_from_angles(alpha, beta, gamma[, k]) Matrix $$p^k D^l(\alpha, \beta, \gamma)$$ D_from_axis_angle(axis, angle) Matrix of the representation, see Irrep.D_from_angles Matrix of the representation, see Irrep.D_from_angles D_from_quaternion(q[, k]) Matrix of the representation, see Irrep.D_from_angles count(_value) Return number of occurrences of value. index(_value) Return first index of value. Equivalent to l == 0 and p == 1 iterator([lmax]) Iterator through all the irreps of $$O(3)$$

Attributes:

 dim The dimension of the representation, $$2 l + 1$$. l The degree of the representation, $$l = 0, 1, \dots$$. p The parity of the representation, $$p = \pm 1$$.
D_from_angles(alpha, beta, gamma, k=None)[source]

Matrix $$p^k D^l(\alpha, \beta, \gamma)$$

(matrix) Representation of $$O(3)$$. $$D$$ is the representation of $$SO(3)$$, see wigner_D.

Parameters
Returns

tensor of shape $$(..., 2l+1, 2l+1)$$

Return type

torch.Tensor

o3.wigner_D, Irreps.D_from_angles

D_from_axis_angle(axis, angle)[source]

Matrix of the representation, see Irrep.D_from_angles

Parameters
Returns

tensor of shape $$(..., 2l+1, 2l+1)$$

Return type

torch.Tensor

D_from_matrix(R)[source]

Matrix of the representation, see Irrep.D_from_angles

Parameters
Returns

tensor of shape $$(..., 2l+1, 2l+1)$$

Return type

torch.Tensor

Examples

>>> m = Irrep(1, -1).D_from_matrix(-torch.eye(3))
>>> m.long()
tensor([[-1,  0,  0],
[ 0, -1,  0],
[ 0,  0, -1]])

D_from_quaternion(q, k=None)[source]

Matrix of the representation, see Irrep.D_from_angles

Parameters
Returns

tensor of shape $$(..., 2l+1, 2l+1)$$

Return type

torch.Tensor

count(_value)[source]

Return number of occurrences of value.

property dim: int[source]

The dimension of the representation, $$2 l + 1$$.

index(_value)[source]

Return first index of value.

Raises ValueError if the value is not present.

is_scalar() bool[source]

Equivalent to l == 0 and p == 1

classmethod iterator(lmax=None)[source]

Iterator through all the irreps of $$O(3)$$

Examples

>>> it = Irrep.iterator()
>>> next(it), next(it), next(it), next(it)
(0e, 0o, 1o, 1e)

property l: int[source]

The degree of the representation, $$l = 0, 1, \dots$$.

property p: int[source]

The parity of the representation, $$p = \pm 1$$.

class e3nn.o3.Irreps(irreps=None)[source]

Bases: tuple

Direct sum of irreducible representations of $$O(3)$$

This class does not contain any data, it is a structure that describe the representation. It is typically used as argument of other classes of the library to define the input and output representations of functions.

dim[source]

the total dimension of the representation

Type

int

num_irreps[source]

number of irreps. the sum of the multiplicities

Type

int

ls[source]

list of $$l$$ values

Type

list of int

lmax[source]

maximum $$l$$ value

Type

int

Examples

Create a representation of 100 $$l=0$$ of even parity and 50 pseudo-vectors.

>>> x = Irreps([(100, (0, 1)), (50, (1, 1))])
>>> x
100x0e+50x1e

>>> x.dim
250


Create a representation of 100 $$l=0$$ of even parity and 50 pseudo-vectors.

>>> Irreps("100x0e + 50x1e")
100x0e+50x1e

>>> Irreps("100x0e + 50x1e + 0x2e")
100x0e+50x1e+0x2e

>>> Irreps("100x0e + 50x1e + 0x2e").lmax
1

>>> Irrep("2e") in Irreps("0e + 2e")
True


Empty Irreps

>>> Irreps(), Irreps("")
(, )


Methods:

 D_from_angles(alpha, beta, gamma[, k]) Matrix of the representation D_from_axis_angle(axis, angle) Matrix of the representation Matrix of the representation D_from_quaternion(q[, k]) Matrix of the representation count(ir) Multiplicity of ir. index(_object) Return first index of value. randn(*size[, normalization, requires_grad, ...]) Random tensor. Remove any irreps with multiplicities of zero. Simplify the representations. List of slices corresponding to indices for each irrep. Sort the representations. spherical_harmonics(lmax[, p]) representation of the spherical harmonics
D_from_angles(alpha, beta, gamma, k=None)[source]

Matrix of the representation

Parameters
Returns

tensor of shape $$(..., \mathrm{dim}, \mathrm{dim})$$

Return type

torch.Tensor

D_from_axis_angle(axis, angle)[source]

Matrix of the representation

Parameters
Returns

tensor of shape $$(..., \mathrm{dim}, \mathrm{dim})$$

Return type

torch.Tensor

D_from_matrix(R)[source]

Matrix of the representation

Parameters

R (torch.Tensor) – tensor of shape $$(..., 3, 3)$$

Returns

tensor of shape $$(..., \mathrm{dim}, \mathrm{dim})$$

Return type

torch.Tensor

D_from_quaternion(q, k=None)[source]

Matrix of the representation

Parameters
Returns

tensor of shape $$(..., \mathrm{dim}, \mathrm{dim})$$

Return type

torch.Tensor

count(ir) int[source]

Multiplicity of ir.

Parameters

ir (e3nn.o3.Irrep) –

Returns

total multiplicity of ir

Return type

int

index(_object)[source]

Return first index of value.

Raises ValueError if the value is not present.

Random tensor.

Parameters
• *size (list of int) – size of the output tensor, needs to contains a -1

• normalization ({'component', 'norm'}) –

Returns

tensor of shape size where -1 is replaced by self.dim

Return type

torch.Tensor

Examples

>>> Irreps("5x0e + 10x1o").randn(5, -1, 5, normalization='norm').shape
torch.Size([5, 35, 5])

>>> random_tensor = Irreps("2o").randn(2, -1, 3, normalization='norm')
>>> random_tensor.norm(dim=1).sub(1).abs().max().item() < 1e-5
True

remove_zero_multiplicities()[source]

Remove any irreps with multiplicities of zero.

Return type

e3nn.o3.Irreps

Examples

>>> Irreps("4x0e + 0x1o + 2x3e").remove_zero_multiplicities()
4x0e+2x3e

simplify()[source]

Simplify the representations.

Return type

e3nn.o3.Irreps

Examples

Note that simplify does not sort the representations.

>>> Irreps("1e + 1e + 0e").simplify()
2x1e+1x0e


Equivalent representations which are separated from each other are not combined.

>>> Irreps("1e + 1e + 0e + 1e").simplify()
2x1e+1x0e+1x1e

slices()[source]

List of slices corresponding to indices for each irrep.

Examples

>>> Irreps('2x0e + 1e').slices()
[slice(0, 2, None), slice(2, 5, None)]

sort()[source]

Sort the representations.

Returns

Examples

>>> Irreps("1e + 0e + 1e").sort().irreps
1x0e+1x1e+1x1e

>>> Irreps("2o + 1e + 0e + 1e").sort().p
(3, 1, 0, 2)

>>> Irreps("2o + 1e + 0e + 1e").sort().inv
(2, 1, 3, 0)

static spherical_harmonics(lmax, p=- 1)[source]

representation of the spherical harmonics

Parameters
• lmax (int) – maximum $$l$$

• p ({1, -1}) – the parity of the representation

Returns

representation of $$(Y^0, Y^1, \dots, Y^{\mathrm{lmax}})$$

Return type

e3nn.o3.Irreps

Examples

>>> Irreps.spherical_harmonics(3)
1x0e+1x1o+1x2e+1x3o

>>> Irreps.spherical_harmonics(4, p=1)
1x0e+1x1e+1x2e+1x3e+1x4e