Antisymmetrizer

In quantum mechanics, an antisymmetrizer Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathcal{A}} (also known as antisymmetrizing operator ) is a linear operator that makes a wave function of N identical fermions antisymmetric under the exchange of the coordinates of any pair of fermions. After application of ${\mathcal {A}}$ the wave function satisfies the Pauli principle. Since Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathcal{A}} is a projection operator, application of the antisymmetrizer to a wave function that is already totally antisymmetric has no effect, Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathcal{A}} is then equal to the identity operator.

Mathematical definition

Consider a wave function depending on the space and spin coordinates of N fermions:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Psi(1,2, \ldots, N)\quad\hbox{with} \quad i \leftrightarrow (\mathbf{r}_i, \sigma_i), }

where the position vector r_i of particle i is a vector in Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathbb{R}^3} and σ_i takes on 2s+1 values, where s is the half-integral intrinsic spin of the fermion. For electrons s = 1/2 and σ can have two values ("spin-up": 1/2 and "spin-down": -1/2). It is assumed that the positions of the coordinates in the notation for Ψ have a well-defined meaning. For instance, the 2-fermion function function Ψ(1,2) will in general be not the same as Ψ(2,1). This implies that in general Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Psi(1,2)- \Psi(2,1) \ne 0 } and therefore we can define meaningfully a transposition operator Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{P}_{ij}} that interchanges the coordinates of particle i and j. In general this operator will not be equal to the identity operator (although in special cases it may be).

A transposition has the parity (also known as signature) -1. The Pauli principle postulates that a wave function of identical fermions must be an eigenfunction of a transposition operator with its parity as eigenvalue

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{P}_{ij} \Psi\big(1,2,\ldots,i, \ldots,j,\ldots, N\big) \equiv \Psi\big(\pi(1),\pi(2),\ldots,\pi(i), \ldots,\pi(j),\ldots, \pi(N)\big) }

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \equiv \Psi(1,2,\ldots,j, \ldots,i,\ldots, N) }

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle = - \Psi(1,2,\ldots,i, \ldots,j,\ldots, N). }

Here we associated the transposition operator Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{P}_{ij}} with the permutation of coordinates π that acts on the set of N coordinates. In this case π = (ij), where (ij) is the cycle notation for the transposition of the coordinates of particle i and j.

Transpositions may be composed (applied in sequence). This defines a product between the transpositions that is associative. It can be shown that an arbitrary permutation of N objects can be written as a product of transpositions and that the number of transposition in this decomposition is of fixed parity. That is, either a permutation is always decomposed in an even number of transpositions (the permutation is called even and has the parity +1), or a permutation is always decomposed in an odd number of transpositions and then it is an odd permutation with parity -1. Denoting the parity of an arbitrary permutation π by (-1)^π, it follows that an antisymmetric wave function satisfies

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{P} \Psi\big(1,2,\ldots, N\big) \equiv \Psi\big(\pi(1),\pi(2),\ldots, \pi(N)\big) = (-1)^\pi \Psi(1,2,\ldots, N), }

where we associated the linear operator Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{P}} with the permutation π.

The set of all N! permutations with the associative product: "apply one permutation after the other", is a group, known as the permutation group or symmetric group, denoted by S_N. After this preamble we are ready to give the definition of the antisymmetrizer

${\mathcal {A}}\equiv {\frac {1}{N!}}\sum _{P\in S_{N}}(-1)^{\pi }{\hat {P}}.$

Properties of the antisymmetrizer

In the representation theory of finite groups the antisymmetrizer is a well-known object, because the set of parities Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \{ (-1)^\pi \}} forms a one-dimensional (and hence irreducible) representation of the permutation group known as the antisymmetric representation. The representation being one-dimensional, the set of parities form the character of the antisymmetric representation. The antisymmetrizer is in fact a character projection operator and is therefore idempotent,

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathcal{A} = \mathcal{A}^2. }

This has the consequence that for any N-particle wave function Ψ(1, ...,N) we have

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathcal{A}\Psi(1,\ldots, N) = \begin{cases} &0 \\ &\Psi'(1,\dots, N) \ne 0. \end{cases} }

Either Ψ does not have an antisymmetric component, and then the antisymmetrizer projects onto zero, or it has one and then the antisymmetrizer projects out this antisymmetric component Ψ'. The antisymmetrizer carries a left and a right representation of the group:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{P} \mathcal{A} = \mathcal{A} \hat{P} = (-1)^\pi \mathcal{A},\qquad \forall \pi \in S_N, }

with the operator Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{P}} representing the coordinate permutation π. Now it holds, for any N-particle wave function Ψ(1, ...,N) with a non-vanishing antisymmetric component, that

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{P} \mathcal{A}\Psi(1,\ldots, N) \equiv \hat{P} \Psi'(1,\ldots, N)=(-1)^\pi \Psi'(1,\ldots, N), }

showing that the non-vanishing component is indeed antisymmetric.

If a wave function is symmetric under any odd parity permutation it has no antisymmetric component. Indeed, assume that the permutation π, represented by the operator Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{P}} , has odd parity and that Ψ is symmetric, then

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{P} \Psi = \Psi \Longrightarrow \mathcal{A} \hat{P} \Psi = \mathcal{A}\Psi \Longrightarrow -\mathcal{A} \Psi = \mathcal{A}\Psi \Longrightarrow \mathcal{A} \Psi = 0. }

As an example of an application of this result, we assume that Ψ is a spin-orbital product. Assume further that a spin-orbital occurs twice (is "doubly occupied") in this product, once with coordinate k and once with coordinate q. Then the product is symmetric under the transposition (k, q) and hence vanishes. Notice that this result gives the original formulation of the Pauli principle: no two electrons can have the same set of quantum numbers (be in the same spin-orbital).

Permutations of identical particles are unitary, (the Hermitian adjoint is equal to the inverse of the operator), and since π and π^-1 have the same parity, it follows that the antisymmetrizer is Hermitian,

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathcal{A}^\dagger = \mathcal{A}. }

The antisymmetrizer commutes with any observable Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{H}\,} (Hermitian operator corresponding to a physical—observable—quantity)

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle [\mathcal{A}, \hat{H}] = 0. }

If it were otherwise, measurement of Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{H}\,} could distinguish the particles, in contradiction with the assumption that only the coordinates of indistinguishable particles are affected by the antisymmetrizer.

Connection with Slater determinant

In the special case that the wave function to be antisymmetrized is a product of spin-orbitals

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Psi(1,2, \ldots, N) = \psi_{n_1}(1) \psi_{n_2}(2) \cdots \psi_{n_N}(N) }

the antisymmetrizer yields a constant times a Slater determinant:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathcal{A} \Psi(1,2, \ldots, N) = \frac{1}{N!} \begin{vmatrix} \psi_{n_1}(1) & \psi_{n_1}(2) & \cdots & \psi_{n_1}(N) \\ \psi_{n_2}(1) & \psi_{n_2}(2) & \cdots & \psi_{n_2}(N) \\ \cdots & \cdots & \cdots & \cdots \\ \psi_{n_N}(1) & \psi_{n_N}(2) & \cdots & \psi_{n_N}(N) \\ \end{vmatrix} }

The correspondence follows immediately from the Leibniz formula for determinants, which reads

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \det(\mathbf{B}) = \sum_{\pi \in S_N} (-1)^\pi B_{1,\pi(1)}\cdot B_{2,\pi(2)}\cdot B_{3,\pi(3)}\cdot\,\cdots\,\cdot B_{N,\pi(N)}, }

where B is the matrix

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathbf{B} = \begin{pmatrix} B_{1,1} & B_{1,2} & \cdots & B_{1,N} \\ B_{2,1} & B_{2,2} & \cdots & B_{2,N} \\ \cdots & \cdots & \cdots & \cdots \\ B_{N,1} & B_{N,2} & \cdots & B_{N,N} \\ \end{pmatrix}. }

To see the correspondence we notice that the fermion labels, permuted by the terms in the antisymmetrizer, label different columns (are second indices). The first indices are orbital indices, n₁, ..., n_N labeling the rows.

Examples

1. By the definition of the antisymmetrizer

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathcal{A} \psi_a(1)\psi_b(2)\psi_c(3) = \frac{1}{6} \Big( \psi_a(1)\psi_b(2)\psi_c(3) + \psi_a(3)\psi_b(1)\psi_c(2) + \psi_a(2)\psi_b(3)\psi_c(1) }

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle -\psi_a(2)\psi_b(1)\psi_c(3) - \psi_a(3)\psi_b(2)\psi_c(1)- \psi_a(1)\psi_b(3)\psi_c(2)\Big). }

Consider the unnormalized Slater determinant

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle D\equiv \begin{vmatrix} \psi_a(1) & \psi_a(2) & \psi_a(3) \\ \psi_b(1) & \psi_b(2) & \psi_b(3) \\ \psi_c(1) & \psi_c(2) & \psi_c(3) \\ \end{vmatrix}. }

By the Laplace expansion along the first row of D

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle D = \psi_a(1) \begin{vmatrix} \psi_b(2) & \psi_b(3) \\ \psi_c(2) & \psi_c(3) \\ \end{vmatrix} -\psi_a(2) \begin{vmatrix} \psi_b(1) & \psi_b(3) \\ \psi_c(1) & \psi_c(3) \\ \end{vmatrix} +\psi_a(3) \begin{vmatrix} \psi_b(1) & \psi_b(2) \\ \psi_c(1) & \psi_c(2) \\ \end{vmatrix}, }

so that

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle D= \psi_a(1)\Big( \psi_b(2) \psi_c(3) - \psi_b(3) \psi_c(2)\Big) - \psi_a(2)\Big( \psi_b(1) \psi_c(3) - \psi_b(3) \psi_c(1)\Big) }

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle + \psi_a(3)\Big( \psi_b(1) \psi_c(2) - \psi_b(2) \psi_c(1)\Big) . }

By comparing terms we see that

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \frac{1} {6} D = \mathcal{A} \psi_a(1)\psi_b(2)\psi_c(3). }

2. It is easy to normalize a projected orbital product Ψ, that is, to compute the normalization constant K from the following:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 1 = K^2 \langle \mathcal{A} \Psi | \mathcal{A} \Psi \rangle =K^2 \langle \Psi | \mathcal{A} \Psi \rangle. }

The rightmost side follows from the hermiticity and idempotency of the antisymmetrizer. Insert the definition of the antisymmetrizer and use the Leibniz rule,

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle K^{-2} = \frac{1}{N!}\sum_{\pi \in S_N} (-1)^\pi \langle \psi_1(1) \psi_2(2) \cdots \psi_N(N)| \psi_1(\pi(1)) \psi_2(\pi(2)) \cdots \psi_N(\pi(N)) \rangle = \det ( \mathbf{S} ) }

where the matrix S, in quantum chemistry known as the overlap matrix, is given by

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathbf{S} =\begin{pmatrix} \langle \psi_1 | \psi_1 \rangle & \langle \psi_1 | \psi_2 \rangle & \langle \psi_1 | \psi_3 \rangle & \cdots & \langle \psi_1 | \psi_N \rangle \\ \langle \psi_2 | \psi_1 \rangle & \langle \psi_2 | \psi_2 \rangle & \cdots & \cdots & \langle \psi_2 | \psi_N \rangle \\ \vdots & \vdots & \ddots & & \vdots \\ \vdots & \vdots & &\ddots & \vdots \\ \langle \psi_N | \psi_1 \rangle & \langle \psi_N | \psi_2 \rangle & \cdots & \cdots & \langle \psi_N | \psi_N \rangle \\ \end{pmatrix} }

The determinant det(S) is the Gram determinant. It is known to vanish if the set Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \{ \psi_i \}} is linearly dependent. In that case K² is undetermined and the projected product is zero. If the set is orthonormal, and a fortiori linearly independent, the matrix S becomes the identity matrix. Such a matrix has unit determinant. In that case K = (N!)^1/2.

Intermolecular antisymmetrizer

One often meets a wave function of the product form Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Psi_A(1,2,\dots,N_A) \Psi_B(N_A+1,N_A+2,\dots,N_A+N_B)} where the total wave function is not antisymmetric, but the factors are antisymmetric,

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathcal{A}^A \Psi_A(1,2,\dots,N_A) = \Psi_A(1,2,\dots,N_A) }

and

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathcal{A}^B\Psi_B(N_A+1,N_A+2,\dots,N_A+N_B) = \Psi_B(N_A+1,N_A+2,\dots,N_A+N_B). }

Here Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathcal{A}^A} antisymmetrizes the first N_A particles and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathcal{A}^B} antisymmetrizes the second set of N_B particles. The operators appearing in these two antisymmetrizers represent the elements of the subgroups S_NA and S_NB, respectively, of S_NA+NB.

Typically, one meets such partially antisymmetric wave functions in the theory of intermolecular forces, where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Psi_A(1,2,\dots,N_A)} is the electronic wave function of molecule A and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Psi_B(N_A+1,N_A+2,\dots,N_A+N_B)} is the wave function of molecule B. When A and B interact, the Pauli principle requires the antisymmetry of the total wave function, also under intermolecular permutations.

The total system can be antisymmetrized by the total antisymmetrizer Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathcal{A}^{AB}} which consists of the (N_A + N_B)! terms in the group S_NA+NB. However, in this way one does not take advantage of the partial antisymmetry that is already present. It is more economic to use the fact that the product of the two subgroups is also a subgroup, and to consider the left cosets of this product group in S_NA+NB:

$S_{N_{A}}\otimes S_{N_{B}}\subset S_{N_{A}+B_{B}}\Longrightarrow \forall \pi \in S_{N_{A}+B_{B}}:\quad \pi =\tau \pi _{A}\pi _{B},\quad \pi _{A}\in S_{N_{A}},\;\;\pi _{B}\in S_{N_{B}},$

where τ is a left coset representative. Since

$(-1)^{\pi }=(-1)^{\tau }(-1)^{\pi _{A}}(-1)^{\pi _{B}},$

we can write

${\mathcal {A}}^{AB}={\tilde {\mathcal {A}}}^{AB}{\mathcal {A}}^{A}{\mathcal {A}}^{B}\quad {\hbox{with}}\quad {\tilde {\mathcal {A}}}^{AB}=\sum _{T=1}^{C_{AB}}(-1)^{\tau }{\hat {T}},\quad C_{AB}={\binom {N_{A}+N_{B}}{N_{A}}}.$

The operator ${\hat {T}}$ represents the coset representative τ (an intermolecular coordinate permutation). Obviously the intermolecular antisymmetrizer ${\tilde {\mathcal {A}}}^{AB}$ has a factor N_A! N_B! less terms then the total antisymmetrizer. Finally,

${\mathcal {A}}^{AB}\Psi _{A}(1,2,\dots ,N_{A})\Psi _{B}(N_{A}+1,N_{A}+2,\dots ,N_{A}+N_{B})$

$={\tilde {\mathcal {A}}}^{AB}\Psi _{A}(1,2,\dots ,N_{A})\Psi _{B}(N_{A}+1,N_{A}+2,\dots ,N_{A}+N_{B}),$

so that we see that it suffices to act with ${\tilde {\mathcal {A}}}^{AB}$ if the wave functions of the subsystems are already antisymmetric.

See also

• Slater determinant
• Identical particles