Lieb–Oxford inequality explained

In quantum chemistry and physics, the Lieb–Oxford inequality provides a lower bound for the indirect part of the Coulomb energy of a quantum mechanical system. It is named after Elliott H. Lieb and Stephen Oxford.

The inequality is of importance for density functional theory and plays a role in the proof of stability of matter.

Introduction

In classical physics, one can calculate the Coulomb energy of a configuration of charged particles in the following way. First, calculate the charge density, where is a function of the coordinates . Second, calculate the Coulomb energy by integrating:

1
2
\int
R3
\int
R3
\rho(x)\rho(y)
|x-y|

d3xd3y.

In other words, for each pair of points and, this expression calculates the energy related to the fact that the charge at is attracted to or repelled from the charge at . The factor of corrects for double-counting the pairs of points.

In quantum mechanics, it is also possible to calculate a charge density, which is a function of . More specifically, is defined as the expectation value of charge density at each point. But in this case, the above formula for Coulomb energy is not correct, due to exchange and correlation effects. The above, classical formula for Coulomb energy is then called the "direct" part of Coulomb energy. To get the actual Coulomb energy, it is necessary to add a correction term, called the "indirect" part of Coulomb energy. The Lieb–Oxford inequality concerns this indirect part. It is relevant in density functional theory, where the expectation value ρ plays a central role.

Statement of the inequality

For a quantum mechanical system of particles, each with charge, the -particle density is denoted by

P(x1,...,xN).

The function is only assumed to be non-negative and normalized. Thus the following applies to particles with any "statistics". For example, if the system is described by a normalised square integrable -particle wave function

\psi\inL2(R3N),

then

P(x1,...,xN)=|\psi(x1,...,x

2.
N)|

More generally, in the case of particles with spin having spin states per particle and with corresponding wave function

\psi(x1,\sigma1,...,xN,\sigmaN)

the -particle density is given by

P(x1,...,xN)=\sum

q|\psi(x
1,\sigma

1,...,xN,\sigma

2.
N)|
Alternatively, if the system is described by a density matrix, then is the diagonal

\gamma(x1,...,xN;x1,...,xN).

The electrostatic energy of the system is defined as

2\sum
I
1\lei<j\leN
\int
R3N
P(x1,...,xi,...,xj,...,xN)
|xi-xj|

d3

3
x
1 … d

xN.

For, the single particle charge density is given by

N\int
\rho(x)=|e|\sum
R3(N-1)

P(x1,...,xi-1,x,xi+1,...,xN)d3

3
x
1 … d

xi-1d3xi+1d3xN

and the direct part of the Coulomb energy of the system of particles is defined as the electrostatic energy associated with the charge density, i.e.

D(\rho)=12\int
R3
\int
R3
\rho(x)\rho(y)
|x-y|

d3xd3y.

The Lieb–Oxford inequality states that the difference between the true energy and its semiclassical approximation is bounded from below as

where is a constant independent of the particle number . is referred to as the indirect part of the Coulomb energy and in density functional theory more commonly as the exchange plus correlation energy. A similar bound exists if the particles have different charges . No upper bound is possible for .

The optimal constant

While the original proof yielded the constant,[1] Lieb and Oxford managed to refine this result to .[2] Later, the same method of proof was used to further improve the constant to .[3] It is only recently that the constant was decreased to .[4] With these constants the inequality holds for any particle number .

The constant can be further improved if the particle number is restricted. In the case of a single particle the Coulomb energy vanishes,, and the smallest possible constant can be computed explicitly as .[2] The corresponding variational equation for the optimal is the Lane–Emden equation of order 3. For two particles it is known that the smallest possible constant satisfies .[2] In general it can be proved that the optimal constants increase with the number of particles, i.e.,[2] and converge in the limit of large to the best constant in the inequality . Any lower bound on the optimal constant for fixed particle number is also a lower bound on the optimal constant . The best numerical lower bound was obtained for where .[5] This bound has been obtained by considering an exponential density. For the same particle number a uniform density gives .

The largest proved lower bound on the best constant is, which was first proven by Cotar and Petrache.[6] The same lower bound was later obtained in using a uniform electron gas, melted in the neighborhood of its surface, by Lewin, Lieb & Seiringer.[7] Hence, to summarise, the best known bounds for are .

The Dirac constant

Historically, the first approximation of the indirect part of the Coulomb energy in terms of the single particle charge density was given by Paul Dirac in 1930 for fermions.[8] The wave function under consideration is

\psi(x1,\sigma1,...,xN,\sigmaN)=

\det(\varphii(xj,\sigmaj))
\sqrt{N!
}. With the aim of evoking perturbation theory, one considers the eigenfunctions of the Laplacian in a large cubic box of volume and sets

\varphi\alpha,k(x,\sigma)=

2\piikx
\chi
\alpha(\sigma)e
\sqrt{|Λ|
},

where forms an orthonormal basis of . The allowed values of are with . For large,, and fixed, the indirect part of the Coulomb energy can be computed to be

EP(Dirac)=-C|e|2/3q-1/3\rho4/3|Λ|,

with .

This result can be compared to the lower bound . In contrast to Dirac's approximation the Lieb–Oxford inequality does not include the number of spin states on the right-hand side. The dependence on in Dirac's formula is a consequence of his specific choice of wave functions and not a general feature.

Generalisations

The constant in can be made smaller at the price of adding another term to the right-hand side. By including a term that involves the gradient of a power of the single particle charge density, the constant can be improved to .[9] [10] Thus, for a uniform density system .

Further reading

Notes and References

  1. Lieb . E. H. . 1979 . A lower bound for Coulomb energies . . 70 . 5–6 . 444–446 . 1979PhLA...70..444L . 10.1016/0375-9601(79)90358-X.
  2. Lieb. E. H. . Oxford . S. . 1981 . Improved lower bound on the indirect Coulomb energy . . 19 . 3 . 427 . 10.1002/qua.560190306.
  3. Kin-Lic Chan . G. . Handy . N. C. . 1999 . Optimized Lieb-Oxford bound for the exchange-correlation energy . . 59 . 4 . 3075 . 1999PhRvA..59.3075K . 10.1103/PhysRevA.59.3075.
  4. Lewin . Mathieu . Lieb . Elliott H. . Seiringer . Robert . Improved Lieb–Oxford bound on the indirect and exchange energies . Letters in Mathematical Physics . October 2022 . 112 . 5 . 92 . 10.1007/s11005-022-01584-5. 2203.12473 . 2022LMaPh.112...92L . 247618886 .
  5. Vuckovic. S.. Gori-Giorgi. P.. 2016. Challenging the Lieb–Oxford bound in a systematic way. Molecular Physics. Molecular Physics. 114. 7–8. 1076–1085. 10.1080/00268976.2015.1136440. Seidl. M.. 1508.01715. 2016MolPh.114.1076S. 100620702.
  6. Cotar. C.. Petrache. M.. 2019. Equality of the Jellium and Uniform Electron Gas next-order asymptotic terms for Coulomb and Riesz potentials. 1707.07664. math-ph.
  7. Lewin. M.. Lieb. E.H.. Seiringer. R.. 2019. Floating Wigner crystal with no boundary charge fluctuations. Phys. Rev. B. 100. 3. 035127. 10.1103/PhysRevB.100.035127. 1905.09138. 2019PhRvB.100c5127L. 162168639.
  8. Dirac . P. A. M. . 2008 . Note on Exchange Phenomena in the Thomas Atom . . 26 . 3 . 376–385 . 1930PCPS...26..376D . 10.1017/S0305004100016108. free .
  9. Benguria . R. D. . Gallegos . P. . Tušek . M. . 2012 . A New Estimate on the Two-Dimensional Indirect Coulomb Energy . . 13 . 8 . 1733 . 2012AnHP...13.1733B . 10.1007/s00023-012-0176-x. 1106.5772 . 119272701 .
  10. Lewin . Mathieu . Lieb . Elliott H. . 2015 . Improved Lieb-Oxford exchange-correlation inequality with a gradient correction . . 91 . 2 . 022507 . 2015PhRvA..91b2507L . 10.1103/PhysRevA.91.022507. 1408.3358 . 119172373 .