# Wigner D-matrix

The Wigner D-matrix is a matrix in an irreducible representation of the groups SU(2) and SO(3). The complex conjugate of the D-matrix is an eigenfunction of the Hamiltonian of spherical and symmetric rigid rotors. The matrix was introduced in 1927 by Eugene Wigner.

## Definition of the Wigner D-matrix

Let Jx, Jy, Jz be generators of the Lie algebra of SU(2) and SO(3). In quantum mechanics these three operators are the components of a vector operator known as angular momentum. Examples are the angular momentum of an electron in an atom, electronic spin, and the angular momentum of a rigid rotor. In all cases the three operators satisfy the following commutation relations,

${\displaystyle [J_{x},J_{y}]=iJ_{z},\quad [J_{z},J_{x}]=iJ_{y},\quad [J_{y},J_{z}]=iJ_{x},}$

where i is the purely imaginary number and Planck's constant ${\displaystyle \hbar }$ has been put equal to one. The operator

${\displaystyle J^{2}=J_{x}^{2}+J_{y}^{2}+J_{z}^{2}}$

is a Casimir operator of SU(2) (or SO(3) as the case may be). It may be diagonalized together with ${\displaystyle J_{z}}$ (the choice of this operator is a convention), which commutes with ${\displaystyle J^{2}}$. That is, it can be shown that there is a complete set of kets with

${\displaystyle J^{2}|jm\rangle =j(j+1)|jm\rangle ,\quad J_{z}|jm\rangle =m|jm\rangle ,}$

where j = 0, 1/2, 1, 3/2, 2,... and m = -j, -j + 1,..., j. For SO(3) the quantum number j is integer.

A rotation operator can be written as

${\displaystyle {\mathcal {R}}(\alpha ,\beta ,\gamma )=e^{-i\alpha J_{z}}e^{-i\beta J_{y}}e^{-i\gamma J_{z}},}$

where α, β, γ are Euler angles (characterized by the keywords: z-y-z convention, right-handed frame, right-hand screw rule, active interpretation).

The Wigner D-matrix is a square matrix of dimension 2j + 1 with general element

${\displaystyle D_{m'm}^{j}(\alpha ,\beta ,\gamma )\equiv \langle jm'|{\mathcal {R}}(\alpha ,\beta ,\gamma )|jm\rangle =e^{-im'\alpha }d_{m'm}^{j}(\beta )e^{-im\gamma }.}$

The matrix with general element

${\displaystyle d_{m'm}^{j}(\beta )=\langle jm'|e^{-i\beta J_{y}}|jm\rangle }$

is known as Wigner's (small) d-matrix.

## Wigner (small) d-matrix

Wigner[1] gave the following expression

${\displaystyle {\begin{array}{lcl}d_{m'm}^{j}(\beta )&=&[(j+m')!(j-m')!(j+m)!(j-m)!]^{1/2}\sum \limits _{s}\left[{\frac {(-1)^{m'-m+s}}{(j+m-s)!s!(m'-m+s)!(j-m'-s)!}}\right.\\&&\left.\cdot \left(\cos {\frac {\beta }{2}}\right)^{2j+m-m'-2s}\left(\sin {\frac {\beta }{2}}\right)^{m'-m+2s}\right].\end{array}}}$

The sum over s is over such values that the factorials are nonnegative.

Note: The d-matrix elements defined here are real. In the often-used z-x-z convention of Euler angles, the factor ${\displaystyle (-1)^{m'-m+s}}$ in this formula is replaced by ${\displaystyle (-1)^{s}\,i^{m-m'}}$, causing half of the functions to be purely imaginary. The realness of the d-matrix elements is one of the reasons that the z-y-z convention, used in this article, is usually preferred in quantum mechanical applications.

The d-matrix elements are related to Jacobi polynomials ${\displaystyle P_{k}^{(a,b)}(\cos \beta )}$ with nonnegative ${\displaystyle a\,}$ and ${\displaystyle b\,}$.[2] Let

${\displaystyle k=\min(j+m,\,j-m,\,j+m',\,j-m').}$

${\displaystyle {\hbox{If}}\quad k={\begin{cases}j+m:&\quad a=m'-m;\quad \lambda =m'-m\\j-m:&\quad a=m-m';\quad \lambda =0\\j+m':&\quad a=m-m';\quad \lambda =0\\j-m':&\quad a=m'-m;\quad \lambda =m'-m\\\end{cases}}}$

Then, with ${\displaystyle b=2j-2k-a\,}$, the relation is

${\displaystyle d_{m'm}^{j}(\beta )=(-1)^{\lambda }{\binom {2j-k}{k+a}}^{1/2}{\binom {k+b}{b}}^{-1/2}\left(\sin {\frac {\beta }{2}}\right)^{a}\left(\cos {\frac {\beta }{2}}\right)^{b}P_{k}^{(a,b)}(\cos \beta ),}$

## Properties of the Wigner D-matrix

The complex conjugate of the D-matrix satisfies a number of differential properties that can be formulated concisely by introducing the following operators with ${\displaystyle (x,\,y,\,z)=(1,\,2,\,3)}$,

${\displaystyle {\begin{array}{lcl}{\hat {\mathcal {J}}}_{1}&=&i\left(\cos \alpha \cot \beta \,{\partial \over \partial \alpha }\,+\sin \alpha \,{\partial \over \partial \beta }\,-{\cos \alpha \over \sin \beta }\,{\partial \over \partial \gamma }\,\right)\\{\hat {\mathcal {J}}}_{2}&=&i\left(\sin \alpha \cot \beta \,{\partial \over \partial \alpha }\,-\cos \alpha \;{\partial \over \partial \beta }\,-{\sin \alpha \over \sin \beta }\,{\partial \over \partial \gamma }\,\right)\\{\hat {\mathcal {J}}}_{3}&=&-i\;{\partial \over \partial \alpha },\end{array}}}$

which have quantum mechanical meaning: they are space-fixed rigid rotor angular momentum operators.

Further,

${\displaystyle {\begin{array}{lcl}{\hat {\mathcal {P}}}_{1}&=&\,i\left({\cos \gamma \over \sin \beta }{\partial \over \partial \alpha }-\sin \gamma {\partial \over \partial \beta }-\cot \beta \cos \gamma {\partial \over \partial \gamma }\right)\\{\hat {\mathcal {P}}}_{2}&=&\,i\left(-{\sin \gamma \over \sin \beta }{\partial \over \partial \alpha }-\cos \gamma {\partial \over \partial \beta }+\cot \beta \sin \gamma {\partial \over \partial \gamma }\right)\\{\hat {\mathcal {P}}}_{3}&=&-i{\partial \over \partial \gamma },\\\end{array}}}$

which have quantum mechanical meaning: they are body-fixed rigid rotor angular momentum operators.

The operators satisfy the commutation relations

${\displaystyle \left[{\mathcal {J}}_{1},\,{\mathcal {J}}_{2}\right]=i{\mathcal {J}}_{3},\qquad {\hbox{and}}\qquad \left[{\mathcal {P}}_{1},\,{\mathcal {P}}_{2}\right]=-i{\mathcal {P}}_{3}}$

and the corresponding relations with the indices permuted cyclically. The ${\displaystyle {\mathcal {P}}_{i}}$ satisfy anomalous commutation relations (have a minus sign on the right hand side).

The two sets mutually commute,

${\displaystyle \left[{\mathcal {P}}_{i},\,{\mathcal {J}}_{j}\right]=0,\quad i,\,j=1,\,2,\,3,}$

and the total operators squared are equal,

${\displaystyle {\mathcal {J}}^{2}\equiv {\mathcal {J}}_{1}^{2}+{\mathcal {J}}_{2}^{2}+{\mathcal {J}}_{3}^{2}={\mathcal {P}}^{2}\equiv {\mathcal {P}}_{1}^{2}+{\mathcal {P}}_{2}^{2}+{\mathcal {P}}_{3}^{2}.}$

Their explicit form is,

${\displaystyle {\mathcal {J}}^{2}={\mathcal {P}}^{2}=-{\frac {1}{\sin ^{2}\beta }}\left({\frac {\partial ^{2}}{\partial \alpha ^{2}}}+{\frac {\partial ^{2}}{\partial \gamma ^{2}}}-2\cos \beta {\frac {\partial ^{2}}{\partial \alpha \partial \gamma }}\right)-{\frac {\partial ^{2}}{\partial \beta ^{2}}}-\cot \beta {\frac {\partial }{\partial \beta }}.}$

The operators ${\displaystyle {\mathcal {J}}_{i}}$ act on the first (row) index of the D-matrix,

${\displaystyle {\mathcal {J}}_{3}\,D_{m'm}^{j}(\alpha ,\beta ,\gamma )^{*}=m'\,D_{m'm}^{j}(\alpha ,\beta ,\gamma )^{*},}$

and

${\displaystyle ({\mathcal {J}}_{1}\pm i{\mathcal {J}}_{2})\,D_{m'm}^{j}(\alpha ,\beta ,\gamma )^{*}={\sqrt {j(j+1)-m'(m'\pm 1)}}\,D_{m'\pm 1,m}^{j}(\alpha ,\beta ,\gamma )^{*}.}$

The operators ${\displaystyle {\mathcal {P}}_{i}}$ act on the second (column) index of the D-matrix

${\displaystyle {\mathcal {P}}_{3}\,D_{m'm}^{j}(\alpha ,\beta ,\gamma )^{*}=m\,D_{m'm}^{j}(\alpha ,\beta ,\gamma )^{*},}$

and because of the anomalous commutation relation the raising/lowering operators are defined with reversed signs,

${\displaystyle ({\mathcal {P}}_{1}\mp i{\mathcal {P}}_{2})\,D_{m'm}^{j}(\alpha ,\beta ,\gamma )^{*}={\sqrt {j(j+1)-m(m\pm 1)}}\,D_{m',m\pm 1}^{j}(\alpha ,\beta ,\gamma )^{*}.}$

Finally,

${\displaystyle {\mathcal {J}}^{2}\,D_{m'm}^{j}(\alpha ,\beta ,\gamma )^{*}={\mathcal {P}}^{2}\,D_{m'm}^{j}(\alpha ,\beta ,\gamma )^{*}=j(j+1)D_{m'm}^{j}(\alpha ,\beta ,\gamma )^{*}.}$

In other words, the rows and columns of the (complex conjugate) Wigner D-matrix span irreducible representations of the isomorphic Lie algebra's generated by ${\displaystyle \{{\mathcal {J}}_{i}\}}$ and ${\displaystyle \{-{\mathcal {P}}_{i}\}}$.

An important property of the Wigner D-matrix follows from the commutation of ${\displaystyle {\mathcal {R}}(\alpha ,\beta ,\gamma )}$ with the time reversal operator ${\displaystyle T\,}$,

${\displaystyle \langle jm'|{\mathcal {R}}(\alpha ,\beta ,\gamma )|jm\rangle =\langle jm'|T^{\,\dagger }{\mathcal {R}}(\alpha ,\beta ,\gamma )T|jm\rangle =(-1)^{m'-m}\langle j,-m'|{\mathcal {R}}(\alpha ,\beta ,\gamma )|j,-m\rangle ^{*},}$

or

${\displaystyle D_{m'm}^{j}(\alpha ,\beta ,\gamma )=(-1)^{m'-m}D_{-m',-m}^{j}(\alpha ,\beta ,\gamma )^{*}.}$

Here we used that ${\displaystyle T\,}$ is anti-unitary (hence the complex conjugation after moving ${\displaystyle T^{\dagger }\,}$ from ket to bra), ${\displaystyle T|jm\rangle =(-1)^{j-m}|j,-m\rangle }$ and ${\displaystyle (-1)^{2j-m'-m}=(-1)^{m'-m}}$.

## Orthogonality relations

The Wigner D-matrix elements ${\displaystyle D_{mk}^{j}(\alpha ,\beta ,\gamma )}$ form a complete set of orthogonal functions of the Euler angles ${\displaystyle \alpha }$, ${\displaystyle \beta ,}$ and ${\displaystyle \gamma }$:

${\displaystyle \int _{0}^{2\pi }d\alpha \int _{0}^{\pi }\sin \beta d\beta \int _{0}^{2\pi }d\gamma \,\,D_{m'k'}^{j'}(\alpha ,\beta ,\gamma )^{\ast }D_{mk}^{j}(\alpha ,\beta ,\gamma )={\frac {8\pi ^{2}}{2j+1}}\delta _{m'm}\delta _{k'k}\delta _{j'j}.}$

This is a special case of the Schur orthogonality relations.

## Kronecker product of Wigner D-matrices, Clebsch-Gordan series

The set of Kronecker product matrices

${\displaystyle {\mathbf {D} }^{j}(\alpha ,\beta ,\gamma )\otimes {\mathbf {D} }^{j'}(\alpha ,\beta ,\gamma )}$

forms a reducible matrix representation of the groups SO(3) and SU(2). Reduction into irreducible components is by the following equation:

${\displaystyle D_{mk}^{j}(\alpha ,\beta ,\gamma )D_{m'k'}^{j'}(\alpha ,\beta ,\gamma )=\sum _{J=|j-j'|}^{j+j'}\sum _{M=-J}^{J}\sum _{K=-J}^{J}\langle jmj'm'|JM\rangle \langle jkj'k'|JK\rangle D_{MK}^{J}(\alpha ,\beta ,\gamma )}$

## Relation to spherical harmonics and Legendre polynomials

For integer values of ${\displaystyle l}$, the D-matrix elements with second index equal to zero are proportional to spherical harmonics and associated Legendre polynomials, normalized to unity and with Condon and Shortley phase convention:

${\displaystyle D_{m0}^{\ell }(\alpha ,\beta ,0)={\sqrt {\frac {4\pi }{2\ell +1}}}Y_{\ell }^{m*}(\beta ,\alpha )={\sqrt {\frac {(\ell -m)!}{(\ell +m)!}}}\,P_{\ell }^{m}(\cos {\beta })\,e^{-im\alpha }}$

This implies the following relationship for the d-matrix:

${\displaystyle d_{m0}^{\ell }(\beta )={\sqrt {\frac {(\ell -m)!}{(\ell +m)!}}}\,P_{\ell }^{m}(\cos {\beta })}$

When both indices are set to zero, the Wigner D-matrix elements are given by ordinary Legendre polynomials:

${\displaystyle D_{0,0}^{\ell }(\alpha ,\beta ,\gamma )=d_{0,0}^{\ell }(\beta )=P_{\ell }(\cos \beta ).}$

In the present convention of Euler angles, ${\displaystyle \alpha }$ is a longitudinal angle and ${\displaystyle \beta }$ is a colatitudinal angle (spherical polar angles in the physical definition of such angles). This is one of the reasons that the z-y-z convention is used frequently in molecular physics. From the time-reversal property of the Wigner D-matrix follows immediately

${\displaystyle \left(Y_{\ell }^{m}\right)^{*}=(-1)^{m}Y_{\ell }^{-m}.}$

There exists a more general relationship to the spin-weighted spherical harmonics:

${\displaystyle D_{-ms}^{\ell }(\alpha ,\beta ,-\gamma )=(-1)^{m}{\sqrt {\frac {4\pi }{2{\ell }+1}}}{}_{s}Y_{{\ell }m}(\beta ,\alpha )e^{is\gamma }.}$

## List of d-matrix elements

Using sign convention of Wigner, et al. the d-matrix elements for j = 1/2, 1, 3/2, and 2 are given below.

for j = 1/2

for j = 1

for j = 3/2

for j = 2 [3]

Wigner d-matrix elements with swapped lower indices are found with the relation:

${\displaystyle d_{m',m}^{j}=(-1)^{m-m'}d_{m,m'}^{j}=d_{-m,-m'}^{j}}$.