Entanglement and Electron-Electron Correlation in Quantum Chemistry

Entanglement as measure of electron–electron correlation in quantum chemistry calculations Abstract In quantum chemistry calculations, the correlation energy is defined as the difference between the Hartree–Fock limit energy and the exact solution of the nonrelativistic Schrödinger equation. With this definition, the electron correlation effects are not directly observable. In this report, we show that the entanglement can be used as an alternative measure of the electron correlation in quantum chemistry calculations. Entanglement is directly observable and it is one of the most striking properties of quantum mechanics. As an example we calculate the entanglement for He atom and H2 molecule with different basis sets.
2005 Elsevier B.V. All rights reserved. The Hartree–Fock self-consistent field approximation, which is based on the idea that we can approximately describe an interacting fermion system in terms of an effective single-particle model, remains the starting point and the major approach for quantitative electronic structure calculations. In quantum chemistry calculations, the correlation energy is defined as the energy error of the Hartree–Fock wave function, i.e., the difference between the Hartree–Fock limit energy and the exact solution of the nonrelativistic Schrödinger equation [1]. There also exists other measures of electron correlation in the literature such as the statistical correlation coefficients [2] and more recently the Shannon entropy as a measure of the correlation strength [3,4]. Electron correlations have a strong influence on many atomic, molecular [5], and solid properties [6]. Recovering the correlation energy for large systems remains one of the most challenging problems in quantum chemistry. The concept of electron correlation as defined in quantum chemistry calculations is useful but not directly observable, i.e., there is no operator in quantum mechanics that its measurement gives the correlation energy. In this Letter, we propose to use the entanglement as a measure of the electron correlation. Entanglement is directly observable and it is one of the most striking properties of quantum mechanics. It was nearly 70 years ago when Schrödinger gave the name ÔentanglementÕ to a correlation of quantum nature. He stated that for an entangled state Ôthe best possible knowledge of the whole does not include the best possible knowledge of its partsÕ [7]. Over the decades the meaning of the word ÔentanglementÕ has changed its flavor and our view of the nature of entanglement may continue to be modified [8]. Entanglement is a quantum mechanical property that describes a correlation between quantum mechanical systems that has no classical analog [9–12]. A pure state of a pair of quantum systems is called entangled if it is unfactorizable, as for example, the singlet state of two spin-1/2 particles, p1ffiffi2 ðj"#i
j#"iÞ. A mixed state is entangled if it can not be represented as a mixture of factorizable pure states [13–15]. Since the 2 seminal work of Einstein, Podolsky, and Rosen [16] there has been a quest for generating entanglement between quantum particles [10,17]. Investigation of quantum entanglement is currently a very active area and has been studied intensely due to its potential applications in quantum communications and information processing [10] such as quantum teleportation [18,19], superdense coding [20], quantum key distribution [21], telecoloning, and decoherence in quantum computers [22–24]. In order to employ the entanglement as an alternative method of measuring the electron correlation, we present the scheme to quantify entanglement based on the entanglement measure for two-particle systems [25,26]. We will obtain a general approach to quantify the entanglement between different spin-orbitals of atomic and molecular systems. For two electron system in 2m-dimensional spinspace orbital with ca and cya denote the fermionic annihilation and creation operators of single-particle states and |0æ represents the vacuum state, a pure two-electron state |Uæ can be written as X xa;b cya cyb j0i; ð1Þ jUi ¼ a;b2f1;2;3;4;...;2mg where a, b run over the orthonormalized single particle states, and Pauli exclusion requires that the 2m · 2m expansion coefficient matrix x is antisymmetric: xa, b =
xb, a, and xi, i = 0. In the occupation number representation (n1›, n1fl, n2›, n2fl, . . . , nm›, nmfl), where › and fl mean a and b electrons, respectively, the subscripts denote the spatial orbital index and m is the total spatial orbital number. By tracing out all other spatial orbitals except n1, we can get a (4 · 4) reduced density matrix for the spatial orbital n1 qn1 ¼ Trn1 jUihUj 0 qn1 ;0 0 B m
1 P B 2 B 0 4 jx2;2iþ1 j B i¼1 ¼B B B 0 0 B @ 0 0 0 0 0 4 m P jx1;2i j i¼2 0 2 B4 B qn1 ;1 ¼ B @ m
1 P jx2;2iþ1 j 2 i¼1 0 ð2Þ qn1 ;2 qn1 ;0 ¼ 4 i¼1 jx2iþ1;2jþ2 j 2 C C C. m P 2A 4 jx1;2i j ð5Þ The matrix elements of x can be calculated from the expansion coefficient of the ab initio Configure Interaction (CI) method. The CI wave function with single and double excitation can be written as X X r;s r;s cra jWra i þ ca;b jWa;b i; ð6Þ jUi ¼ c0 jW0 i þ ar a 3 Å the correlation between the two electrons is almost zero but the entanglement is maximal until around R  4 Å, the entanglement vanishes for R > 4 Å. To understand the entanglement behavior for H2 molecule using ab initio quantum chemistry methods, we calculate the entanglement for a simpler two-electron model system. This is a model of two spin-1/2 electrons with an exchange coupling constant J (a.u.) in an effective transverse magnetic field of strength B (a.u.). In order to describe the environment of the electrons in a molecule, we simply introduce a small effective external magnetic field B. The general Hamiltonian for such a system is given by J J H ¼
ð1 þ cÞrx1  rx2
ð1
cÞry1  ry2
Brz1 2 2 z  I 2
BI 1  r2 ; ð12Þ where ra are the Pauli matrices (a = x, y, z) and c is the degree of anisotropy. For c = 1 Eq. (12) reduces to the Ising model, whereas for c = 0 it is the XY model. Our two spin problem admits an exact solution, it is simply a (4 · 4) matrix with the following four eigenvalues: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k1 ¼
J ; k2 ¼ J ; k3 ¼
4B2 þ J 2 c2 ; qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð13Þ k4 ¼ 4B2 þ J 2 c2 and the corresponding eigenvectors: 0 1 0 1 0 0 p ffiffi ffi p ffiffi ffi B 1= 2 C B
1= 2 C B B C pffiffiffi C j/1 i ¼ B pffiffiffi C; j/2 i ¼ B C; @ 1= 2 A @ 1= 2 A 0 0 qffiffiffiffiffiffiffiffi 1 aþ2B 2a C B B 0 C B C j/3 i ¼ B C; B 0 C @ qffiffiffiffiffiffiffiffi A a
2B 2a 0 0 qffiffiffiffiffiffiffiffi 1
a
2B 2a C B B C 0 B C j/4 i ¼ B C; B C 0 @ qffiffiffiffiffiffiffiffi A ð14Þ aþ2B 2a pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi where a ¼ 4B2 þ J 2 c2 . In the basis set {|››æ, |›flæ, |fl›æ, |flflæ}, the eigenvectors can be written as 1 j/1 i ¼ pffiffiffi ðj#"iþ j"#iÞ; 2 1 j/2 i ¼ pffiffiffi ðj#"i
j"#iÞ; 2 rffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffi a
2B a þ 2B j/3 i ¼ j##i þ j""i; 2a 2a rffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffi a þ 2B a
2B j##i
j""i. j/4 i ¼ 2a 2a ð15Þ ð16Þ ð17Þ ð18Þ Now we confine our interest to the calculation of the entanglement between the two electronic spins. For simplicity we take c = 1, Eq. (12) reduces to the Ising model with the ground state energy k3 and the corresponding eigenvector j/3æ. All the information needed for quantifying the entanglement in this case is contained in the two-electron density matrix. When a biparticle quantum system AB is in a pure state there is essentially a unique measure of the entanglement between the subsystems A and B given by the von Neumann entropy S [29]. If we denote qA the partial trace of qAB with respect to subsystem B, qA = TrB(qAB), the entanglement of the state qAB is defined as the von Neumann entropy of the reduced density operator qA, S(qAB) ”
Tr[qA log2qA]. For our model system in the ground state j/3æ, the reduced density matrix in the basis set (›,fl) is given by ! aþ2h 0 2a . ð19Þ qA ¼ 0 a
2h 2a Thus, the entanglement is simply given by 4 pffiffiffiffiffiffiffiffiffiffiffiffiffi   4 þ k2
2 1 1 1 1 ffiffiffiffiffiffiffiffiffiffiffiffi ffi p p ffiffiffiffiffiffiffiffiffiffiffiffi ffi ; þ
S ¼
log2 log 2 2 4 4 þ k2 4 þ k2 þ 2 4 þ k2 ð20Þ where k = J/B. The value of J, the exchange coupling constant between the spins of the two electrons, can be calculated as half the energy difference between the lowest singlet and triplet states of the hydrogen molecule. Herring and Flicker have shown [30] that J for H2 molecule can be approximated as a function of the interatomic distance R. In atomic units, the expression for large R is given by J ðRÞ ¼
0.821R5=2 e
2R þ OðR2 e
2R Þ. ð21Þ Fig. 2 shows the calculated von Neumann Entanglement (S), Eq. (20), as a function of the distance between the two electronic spins R, using J(R) of Eq. (21), for different values of the magnetic field strength B. At the limit R ! 1 the exchange interaction J vanishes as a result the two electronic spins are up and the wave function is factorazable, i.e., the entanglement is zero. At the other limit, when R = 0 the entanglement is zero for this model because J = 0. As R increases, the exchange interaction increases leading to increasing entanglement between the two electronic spins. However this increase in the entanglement reaches a maximum limit as shown in the figure. For large distance, the exchange interaction decreases exponentially with R and thus the decrease of the entanglement. The figure also shows that the entanglement increases with decreasing the magnetic field strength. This can be attributed to effectively increasing the exchange interaction. Thus, we get the similar behavior as the entanglement for the H2 molecule as a function of the internuclear distance R, using accurate ab initio methods. Because the Eq. (21) is only applicable to the large value of R, it is not surprising to see, in the limit R ! 0, the entanglement converges to the He atom results in ab initio methods but disappears in this simple model. Recently, a new promising approach is emerging for the realization of quantum chemistry calculations without wave functions through first order semidefinite programming [31]. The electronic energies and properties of atoms and molecules are computable simply from an effective two-electron reduced density matrix q(AB). Thus, the electron correlation can be directly calculated as effectively with the entanglement between the two electrons, which is readily calculated as the von Neumann entropy S =
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