On Kohn-Sham models with LDA and GGA exchange-correlation functionals

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On Kohn-Sham models with LDA and GGA exchange-correlation functionals

Arnaud Anantharaman, Eric Cancès

To cite this version:

Arnaud Anantharaman, Eric Cancès. On Kohn-Sham models with LDA and GGA exchange- correlation functionals. 2008. �inria-00325660�

On Kohn-Sham models with LDA and GGA exchange-correlation functionals

Arnaud Anantharaman and Eric Canc`es

CERMICS, Ecole des Ponts and Universit´e Paris Est 6 & 8 avenue Blaise Pascal, 77455 Marne-la-Vall´ee Cedex 2, France

and

INRIA Rocquencourt, Micmac Project Team

Domaine de Voluceau, B.P. 105, 78153 Le Chesnay Cedex, France

29th September 2008

Abstract

This article is concerned with the mathematical analysis of the Kohn-Sham and extended Kohn-Sham models, in the local density approximation (LDA) and gener- alized gradient approximation (GGA) frameworks. After recalling the mathematical derivation of the Kohn-Sham and extended Kohn-Sham LDA and GGA models from the Schr¨odinger equation, we prove that the extended Kohn-Sham LDA model has a solution for neutral and positively charged systems. We then prove a similar result for the spin-unpolarized Kohn-Sham GGA model for two-electron systems, by means of a concentration-compactness argument.

1 Introduction

Density Functional Theory (DFT) is a powerful, widely used method for computing ap- proximations of ground state electronic energies and densities in chemistry, materials sci- ence, biology and nanosciences.

According to DFT [10, 15], the electronic ground state energy and density of a given molecular system can be obtained by solving a minimization problem of the form

inf

F(ρ) + Z

R3

ρV, ρ≥0, √ρ∈H¹(R³), Z

R3

ρ=N

whereN is the number of electrons in the system,V the electrostatic potential generated by the nuclei, and F some functional of the electronic density ρ, the functional F being universal, in the sense that it does not depend on the molecular system under consid- eration. Unfortunately, no tractable expression for F is known, which could be used in numerical simulations.

The groundbreaking contribution which turned DFT into a useful tool to perform cal- culations, is due to Kohn and Sham [11], who introduced the local density approximation (LDA) to DFT. The resulting Kohn-Sham LDA model is still commonly used, in par- ticular in solid state physics. Improvements of this model have then been proposed by many authors, giving rise to Kohn-Sham GGA models [12, 21, 2, 20], GGA being the abbreviation of Generalized Gradient Approximation. While there is basically a unique Kohn-Sham LDA model, there are several Kohn-Sham GGA models, corresponding to

different approximations of the so-called exchange-correlation functional. A given GGA model will be known to perform well for some classes of molecular system, and poorly for some other classes. In some cases, the best result will be obtained with LDA. It is to be noticed that each Kohn-Sham model exists in two versions: the standard version, with integer occupation numbers, and the extended version with “fractional” occupation numbers. As explained below, the former one originates from Levy-Lieb’s (pure state) contruction of the density functional, while the latter is derived from Lieb’s (mixed state) construction.

To our knowledge, there are very few results on Kohn-Sham LDA and GGA models in the mathematical literature. In fact, we are only aware of a proof of existence of a minimizer for the standard Kohn-Sham LDA model by Le Bris [13]. In this contribution, we prove the existence of a minimizer for the extended Kohn-Sham LDA model, as well as for the two-electron standard and extended Kohn-Sham GGA models, under some conditions on the GGA exchange-correlation functional.

Our article is organized as follows. First, we provide a detailed presentation of the various Kohn-Sham models, which, despite their importance in physics and chemistry [24], are not very well known in the mathematical community. The mathematical foundations of DFT are recalled in section 2, and the derivation of the (standard and extended) Kohn- Sham LDA and GGA models is discussed in section 3. We state our main results in section 4, and postpone the proofs until section 5.

We restrict our mathematical analysis to closed-shell, spin-unpolarized models. All our results related to the LDA setting can be easily extended to open-shell, spin-polarized models (i.e. to the local spin-density approximation LSDA). Likewise, we only deal with all electron descriptions, but valence electron models with usual pseudo-potential approx- imations (norm conserving [29], ultrasoft [30], PAW [3]) can be dealt with in a similar way.

2 Mathematical foundations of DFT

As mentioned previously, DFT aims at calculating electronic ground state energies and densities. Recall that the ground state electronic energy of a molecular system composed of M nuclei of charges z₁, ..., z_M (z_k ∈ N\ {0} in atomic units) and N electrons is the bottom of the spectrum of the electronic hamiltonian

H_N =−1 2

XN

i=1

∆r_i− XN

i=1

XM

k=1

z_k

|r_i−R_k|+ X

1≤i<j≤N

1

|r_i−r_j| (1) whereriandR_kare the positions inR³of thei^thelectron and thek^thnucleus respectively.

The hamiltonian H_N acts on electronic wavefunctions Ψ(r₁, σ₁;· · ·;r_N, σ_N), σ_i ∈ Σ :=

{|↑i,|↓i}denoting the spin variable of thei^thelectron, the nuclear coordinates{R_k}1≤k≤M

playing the role of parameters. It is convenient to denote by R³

Σ := R³ × {|↑i,|↓i} and x_i := (r_i, σ_i). As electrons are fermions, electonic wavefunctions are antisymmetric with respect to the renumbering of electrons, i.e.

Ψ(x_p(1),· · ·,x_p(N)) =ǫ(p)Ψ(x1,· · · ,xN)

where ǫ(p) is the signature of the permutation p. Note that (in the absence of magnetic fields) H_NΨ is real-valued if Ψ is real-valued. Our purpose being the calculation of the bottom of the spectrum ofH_N, there is therefore no restriction in considering real-valued

wavefunctions only. In other words,H_N can be considered here as an operator on the real Hilbert space

HN =

^N

i=1

L²(R³

Σ), endowed with the inner product

hΨ|Ψ^′iHN = Z

(R3 Σ)^N

Ψ(x₁,· · · ,x_N) Ψ^′(x₁,· · · ,x_N)dx₁· · ·dx_N,

where Z

R3 Σ

f(x)dx:=X

σ∈Σ

Z

R3

f(r, σ)dr, and the corresponding norm k · kHN =h·|·i

1 2

HN. It is well-known that H_N is a self-adjoint operator on HN with form domain

Q^N =

^N

i=1

H¹(R³_Σ).

Denoting by

Z = XM

k=1

z_k

the total nuclear charge of the system, it results from Zhislin’s theorem that for neutral or positively charged systems (Z ≥N), H_N has an infinite number of negative eigenvalues below the bottom of its essential spectrum. In particular, the electronic ground state energyλ₁(H_N) is an eigenvalue of H_N, and more precisely the lowest one.

In any case, i.e. whatever Z and N, we always have

λ₁(H_N) = inf{hΨ|H_N|Ψi, Ψ∈ QN, kΨkHN = 1}. (2) Note that it also holds

λ₁(H_N) = inf{Tr (H_NΓ), Γ∈ S(HN), Ran(Γ)⊂ QN, 0≤Γ≤1, Tr (Γ) = 1}. (3) In the above expression, S(HN) is the vector space of bounded self-adjoint operators on HN, and the condition 0≤Γ≤1 stands for 0≤ hΨ|Γ|Ψi ≤ kΨk²HN for all Ψ∈ HN. Note that if H is a bounded-from-below self-adjoint operator on some Hilbert space H, with form domainQ, and ifDis a positive trace-class self-adjoint operator onH, Tr (HD) can always be defined inR₊∪ {+∞}as Tr (HD) = Tr ((H−a)¹²D(H−a)¹²) +aTr (D) where ais any real number such thatH ≥a.

From a physical viewpoint, (2) and (3) mean that the ground state energy can be computed either by minimizing over pure states (characterized by wavefunctions Ψ) or by minimizing over mixed states (characterized by density operators Γ).

With any N-electron wavefunction Ψ∈ HN such thatkΨkHN = 1 can be associated the electronic density

ρ_Ψ(r) =N X

σ∈Σ

Z

(R³

Σ)^N−1|Ψ(r, σ;x₂,· · ·;x_N)|²dx₂· · ·dx_N.

Likewise, one can associate with any N-electron density operator Γ ∈ S(HN) such that 0≤Γ≤1 and Tr (Γ) = 1, the electronic density

ρ_Γ(r) =N X

σ∈Σ

Z

(R³

Σ)^N−1

Γ(r, σ;x2,· · · ,xN;r, σ;x2,· · ·;xN)dx2· · ·dxN

(here and below, we use the same notation for an operator and its Green kernel).

Let us denote by

V(r) =− XM

k=1

z_k

|r−R_k| the electrostatic potential generated by the nuclei, and by

H_N¹ =−1 2

XN

i=1

∆r_i+ X

1≤i<j≤N

1

|r_i−r_j|. (4) It is easy to see that

hΨ|H_N|Ψi=hΨ|H_N¹|Ψi+ Z

R3

ρ_ΨV and Tr (HNΓ) = Tr (H_N¹Γ) + Z

R3

ρ_ΓV.

Besides, it can be checked that

R^N = {ρ| ∃Ψ∈ Q^N,kΨk^HN = 1, ρΨ =ρ}

= {ρ| ∃Γ∈ S(HN),Ran(Γ)⊂ QN, 0≤Γ≤1, Tr (Γ) = 1, ρ_Γ=ρ}

=

ρ≥0|√ρ ∈H¹(R³), Z

R3

ρ=N

.

It therefore follows that

I_N = inf

F_LL(ρ) + Z

R3

ρV, ρ∈ RN

(5)

= inf

F_L(ρ) + Z

R3

ρV, ρ∈ RN

(6) where Levy-Lieb’s and Lieb’s density functionals [14, 15] are respectively defined by

F_LL(ρ) = inf

hΨ|H_N¹|Ψi, Ψ∈ QN, kΨkHN = 1, ρ_Ψ=ρ (7) F_L(ρ) = inf

Tr (H_N¹Γ), Γ∈ S(H^N), Ran(Γ)⊂ Q^N,

0≤Γ≤1, Tr (Γ) = 1, ρ_Γ =ρ . (8) Note that the functionalsF_LLandF_Lare independent of the nuclear potentialV, i.e. they do not depend on the molecular system. They are therefore universal functionals of the density. It is also shown in [15] thatF_L is the Legendre transform of the functionV 7→I_N. More precisely, expliciting the dependency of I_N onV, it holds

F_L(ρ) = sup

I_N(V)− Z

R3

ρV, V ∈L³²(R³) +L^∞(R³)

,

from which it follows in particular that F_L is convex on the convex set RN (and can be extended to a convex functional onL¹(R³)∩L³(R³)).

Formulae (5) and (6) show that, in principle, it is possible to compute the electronic ground state energy (and the corresponding groud state density if it exists) by solving a

minimization problem on RN. At this stage no approximation has been made. But, as neither F_LL norF_L can be easily evaluated for the real system of interest (N interacting electrons), approximations are needed to make of the density functional theory a practi- cal tool for computing electronic ground states. Approximations rely on exact, or very accurate, evaluations of the density functional for reference systems “close” to the real system:

• in Thomas-Fermi and related models, the reference system is an homogeneous elec- tron gas;

• in Kohn-Sham models (by far the most commonly used), it is a system of N non- interacting electrons.

3 Kohn-Sham models

For a system of N non-interacting electrons, universal density functionals are obtained as explained in the previous section; it suffices to replace the interacting hamiltonianH_N¹ of the physical system (formula (4)) with the hamiltonian of the reference system

H_N⁰ =− XN

i=1

1

2∆r_i. (9)

The analogue of the Levy-Lieb density functional (7) then is the Kohn-Sham type kinetic energy functional

Te_KS(ρ) = inf

hΨ|H_N⁰|Ψi, Ψ∈ Q^N, kΨk^HN = 1, ρΨ=ρ , (10) while the analogue of the Lieb functional (8) is the Janak kinetic energy functional

T_J(ρ) = inf

Tr (H_N⁰Γ), Γ∈ S(HN), Ran(Γ)⊂ QN, 0≤Γ≤1, Tr (Γ) = 1, ρ_Γ=ρ . Let Γ be in the above minimization set. The energy Tr (H_N⁰Γ) can be rewritten as a function of the one-electron reduced density operator Υ_Γ associated with Γ. Recall that Υ_Γ is the self-adjoint operator onL²(R³

Σ) with kernel Υ_Γ(x,x^′) =N

Z

(R³

Σ)^N−1

Γ(x,x₂,· · · ,x_N;x^′,x₂,· · · ,x_N)dx₂· · ·dx_N.

Indeed, a simple calculation yields Tr (H_N⁰Γ) = Tr (−¹₂∆rΥ_Γ), where ∆r is the Laplace operator on L²(R³

Σ) - acting on the space coordinate r. Besides, it is known (see e.g. [5]) that

{Υ| ∃Γ∈ S(HN), Ran(Γ)⊂ QN, 0≤Γ≤1, Tr (Γ) = 1, Υ_Γ= Υ, ρ_Γ =ρ}

=

Υ∈ S(L²(R³

Σ)), 0≤Υ≤1, Ran(Υ)⊂H¹(R³

Σ),Tr (Υ) =N, ρ_Υ=ρ , (11) where

ρ_Υ(r) := X

σ∈Σ

Υ(r, σ;r, σ).

Hence,

T_J(ρ) = inf

Tr

−1 2∆rΥ

, Υ∈ S(L²(R³

Σ)), 0≤Υ≤1, Ran(Υ)⊂H¹(R³_Σ),Tr (Υ) =N, ρ_Υ =ρ

. (12)

It is to be noticed that no such simple expression for Te_KS(ρ) is available because one lacks anN-representation result similar to (11) for pure state one-particle reduced density operators. In the standard Kohn-Sham model, Te_KS(ρ) is replaced with the Kohn-Sham kinetic energy functional

T_KS(ρ) = inf

hΨ|H_N⁰|Ψi, Ψ∈ QN, Ψ is a Slater determinant, ρ_Ψ=ρ , (13) where we recall that a Slater determinant is a wavefunction Ψ of the form

Ψ(x1,· · ·,xN) = 1

√N!det(φi(xj)) with φ_i ∈L²(R³_Σ), Z

R3

φ_i(x)φj(x)dx=δ_ij. It is then easy to check that

T_KS(ρ) = inf (1

2 XN

i=1

Z

R3 Σ

|∇φ_i(x)|²dx, Φ = (φ1,· · · , φ_N)∈ W^N, ρ_Φ=ρ )

, (14) where we have set

WN = (

Φ = (φ₁,· · · , φ_N) | φ_i ∈H¹(R³_Σ), Z

R³

Σ

φ_i(x)φ_j(x)dx=δ_ij )

and

ρ_Φ(r) = XN

i=1

X

σ∈Σ

|φ_i(r, σ)|². Note that for an arbitraryρ∈ RN, it holds

T_J(ρ)≤Te_KS(ρ)≤T_KS(ρ).

It is not difficult to check that (12) always has a minimizer. If one of the minimizers Υ of (12) is of rank N, then Υ =PN

i=1|φ_iihφ_i|with Φ = (φ₁,· · ·, φ_N) ∈ WN, Φ being then a minimizer of (13) andT_KS(ρ) =T_J(ρ). Otherwise,T_KS(ρ)> T_J(ρ).

The density functionals T_KS and T_J associated with the non interacting hamiltonian H₀ are expected to provide acceptable approximations of the kinetic energy of the real (interacting) system. Likewise, the Coulomb energy

J(ρ) = 1 2

Z

R3

Z

R3

ρ(r)ρ(r^′)

|r−r^′| drdr^′

representing the electrostatic energy of a classical charge distribution of density ρ is a reasonable guess for the electronic interaction energy in a system ofN electrons of density ρ. The errors on both the kinetic energy and the electrostatic interaction are put together in theexchange-correlation energy defined as the difference

E_xc(ρ) =F_LL(ρ)−T_KS(ρ)−J(ρ), (15) or

E_xc(ρ) =F_L(ρ)−T_J(ρ)−J(ρ), (16) depending on the choices for the interacting and non-interacting density functionals. We finally end up with the so-called Kohn-Sham and extended Kohn-Sham models

I_N^KS = inf 1

2 XN

i=1

Z

R³

Σ

|∇φ_i(x)|²dx+ Z

R3

ρ_ΦV +J(ρΦ) +E_xc(ρΦ), Φ = (φ₁,· · ·, φ_N)∈ WN

, (17)

and

I_N^EKS = inf

Tr

−1 2∆rΥ

+

Z

R3

ρ_ΥV +J(ρΥ) +E_xc(ρΥ),

Υ∈ S(L²(R³_Σ)), 0≤Υ≤1, Tr (Υ) =N, Tr (−∆rΥ)<∞

, (18) the condition on Tr (−∆rΥ) ensuring that each term of the energy functional is well- defined.

Up to now, no approximation has been made, in such a way that for the exact exchange- correlation functionals ((15) or (16)), I_N^KS = I_N^EKS = λ₁(HN) for all molecular system containingN electrons. Unfortunately, there is no tractable expression ofE_xc(ρ) that can be used in numerical simulations. Before proceeding further, and for the sake of simplicity, we will restrict ourselves to closed-shell, spin-unpolarized, systems. This means that we will only consider molecular systems with an even number of electrons N = 2N_p, where N_p is the number of electron pairs in the system, and that we will assume that electrons

“go by pairs”. In the Kohn-Sham formalism, this means that the set of admissible states reduces to

Φ = (ϕ1α, ϕ₁β,· · · , ϕ_Npα, ϕ_Npβ)|ϕ_i ∈H¹(R³), Z

R3

ϕ_iϕ_j =δ_ij

whereα(|↑i) = 1, α(|↓i) = 0,β(|↑i) = 0 andβ(|↓i) = 1, yielding the spin-unpolarized (or closed-shell, or restricted) Kohn-Sham model

I_N^RKS = inf X^Np

i=1

Z

R3|∇ϕ_i|²+ Z

R3

ρ_ΦV +J(ρΦ) +E_xc(ρΦ),

Φ= (ϕ₁,· · ·, ϕ_Np)∈(H¹(R³))^Np, Z

R3

ϕ_iϕ_j =δ_ij, ρ_Φ = 2

Np

X

i=1

|ϕ_i|²

, (19) where the factor 2 in the definition ofρ_Φ accounts for the spin. Likewise, the constraints on the one-electron reduced density operators originating from the closed-shell approximation read:

Υ(r,|↑i,r^′,|↑i) = Υ(r,|↓i,r^′,|↓i) and Υ(r,|↑i,r^′,|↓i) = Υ(r,|↓i,r^′,|↑i) = 0.

Introducing γ(r,r^′) = Υ(r,|↑i,r^′,|↑i) and denoting by ρ_γ(r) = 2γ(r,r), we obtain the spin-unpolarized extended Kohn-Sham model

I_N^REKS=

E(γ), γ ∈ KNp

where

E(γ) = Tr (−∆γ) + Z

R3

ρ_γV +J(ρ_γ) +E_xc(ρ_γ), and

KNp=

γ ∈ S(L²(R³))|0≤γ ≤1, Tr (γ) =N_p, Tr (−∆γ)<∞ . Note that any γ ∈ KNp is of the form

γ = X+∞

i=1

n_i|φ_iihφ_i|

with

φ_i ∈H¹(R³), Z

R3

φ_iφ_j=δ_ij, n_i∈[0,1],

+∞X

i=1

n_i =N_p,

+∞X

i=1

n_ik∇φ_ik²_L² <∞.

In particular,

ρ_γ(r) = 2 X+∞

i=1

n_i|φ_i(r)|².

Let us also remark that problem (19) can be recast in terms of density operators as follows I_N^RKS=

E(γ), γ ∈ KNp (20)

where

P^Np=

γ ∈ S(L²(R³))|γ² =γ, Tr (γ) =N_p, Tr (−∆γ)<∞

is a the set of finite energy rank-Np orthogonal projectors (note that K^Np is the convex hull of PNp). The connection between (19) and (20) is given by the correspondence

γ =

Np

X

i=1

|φ_iihφ_i|,

i.e. γ is the orthogonal projector on the vector space spanned by the φ_i. Indeed, as

|∇|= (−∆)¹², it holds

Tr (−∆γ) = Tr (|∇|γ|∇|) =

Np

X

i=1

k|∇|φ_ik²L² =

Np

X

i=1

k∇φ_ik²L² =

Np

X

i=1

Z

R3|∇φ_i|².

Let us now address the issue of constructing relevant approximations for E_xc(ρ). In their celebrated 1964 article, Kohn and Sham proposed to use an approximate exchange- correlation functional of the form

E_xc(ρ) = Z

R3

g(ρ(r))dr (LDA exchange-correlation functional) (21) whereρ⁻¹g(ρ) is the exchange-correlation density for a uniform electron gas with density ρ, yielding the so-called local density approximation (LDA). In practical calculations, it is made use of approximations of the function ρ 7→ g(ρ) (from R₊ to R) obtained by interpolating asymptotic formulae for the low and high density regimes (see e.g. [6]) and accurate quantum Monte Carlo evaluations of g(ρ) for a small number of values of ρ [4].

Several interpolation formulae are available [23, 22, 31], which provide similar results. In the 80’s, refined approximations ofE_xchave been constructed, which take into account the inhomogeneity of the electronic density in real molecular systems. Generalized gradient approximations (GGA) of the exchange-correlation functional are of the form

E_xc(ρ) = Z

R3

h(ρ(r),1 2|∇p

ρ(r)|²)dx (GGA exchange-correlation functional). (22) Contrarily to the situation encountered for LDA, the function (ρ, κ) 7→ g(ρ, κ) (from R₊×R₊ to R) does not have a univoque definition. Several GGA functionals have been proposed and new ones come up periodically.

Remark 1. We have chosen the form (22) for the GGA exchange-correlation functional because it is well suited for the study of spin-unpolarized two electron systems (see The- orem 2 below). In the Physics literature, spin-unpolarized LDA and GGA exchange- correlation functionals are rather written as follows

E_xc(ρ) =E_x(ρ) +E_c(ρ) with

E_x(ρ) = Z

R3

ρ(r)ǫ_x(ρ(r))F_x(s_ρ(r))dr (23) E_c(ρ) =

Z

R3

ρ(r) [ǫ_c(r_ρ(r)) +H(r_ρ(r), t_ρ(r))] dr. (24) In the above decomposition, E_x is the exchange energy, E_c is the correlation energy, ǫ_x and ǫ_c are respectively the exchange and correlation energy densities of the homogeneous electron gas, r_ρ(r) = ⁴₃πρ(r)−¹₃

is the Wigner-Seitz radius, s_ρ(r) = ¹

2(3π²)¹³

|∇ρ(r)|

ρ(r)⁴³ is the (non-dimensional) reduced density gradient, t_ρ(r) = ¹

4(3π⁻¹)¹⁶

|∇ρ(r)|

ρ(r)⁷⁶ is the correlation gra- dient, F_x is the so-called exchange enhancement factor, andH is the gradient contribution to the correlation energy. Whileǫ_x has a simple analytical expression, namely

ǫ_x(ρ) =−3 4

3 π

¹₃ ρ¹³

ǫ_c has to be approximated (as explained above for the functiong). For LDA,F_x is every- where equal to one and H = 0. A popular GGA exchange-correlation energy is the PBE functional [20], for which

F_x(s) = 1 + µs² 1 +µν⁻¹s² H(r, t) = θln

1 +υ

θt² 1 +A(r)t² 1 +A(r)t²+A(r)²t⁴

with A(r) = υ θ

e^−ǫc(r)/θ −1−1

, the values of the parameters µ ≃ 0.21951, ν ≃ 0.804, θ = π⁻²(1−ln 2) and υ = 3π⁻²µ following from theoretical arguments.

4 Main results

Let us first set up and comment on the conditions on the LDA and GGA exchange- correlation functionals under which our results hold true:

• the function g in (21) is a C¹ function fromR₊ to R, twice differentiable and such that

g(0) = 0 (25)

g^′≤0 (26)

∃0< β₋ ≤β₊< 2

3 s.t. sup

ρ∈R₊

|g^′(ρ)|

ρ^β−+ρ^β+ <∞ (27)

∃1≤α < 3

2 s.t. lim sup

ρ→0⁺

g(ρ)

ρ^α <0; (28)

• the functionh in (21) is aC¹ function fromR₊×R₊ to R, twice differentiable with respect to the second variable, and such that

h(0, κ) = 0, ∀κ∈R₊ (29)

∂h

∂ρ ≤0 (30)

∃0< β₋≤β₊< 2

3 s.t. sup

(ρ,κ)∈R₊×R₊

∂h

∂ρ(ρ, κ)

ρ^β−+ρ^β+ <∞ (31)

∃1≤α < 3

2 s.t. lim sup

(ρ,κ)→(0⁺,0⁺)

h(ρ, κ)

ρ^α <0 (32)

∃0< a≤b <∞ s.t. ∀(ρ, κ)∈R₊×R₊, a≤1 +∂h

∂κ(ρ, κ)≤b (33)

∀(ρ, κ)∈R₊×R₊, 1 + ∂h

∂κ(ρ, κ) + 2κ∂²h

∂κ²(ρ, κ)≥0. (34) Conditions (25)-(28) on the LDA exchange-correlation energy are not restrictive. They are obviously fulfilled by the LDA exchange functional (g_x^LDA(ρ) =−³_{4 π}³¹₃

ρ⁴³), and are also satisfied by all the approximate LDA correlation functionals currently used in practice (withα= ⁴₃ and β₋=β⁺= ¹₃). We have checked numerically that assumptions (29)-(34) are satisfied by the PZ81 functional defined in [23].

Remark 2. Our results remain true if (26) and (30) are respectively replaced with the weaker conditions

∃1

3 ≤β₋^′ ≤β₊ < 2

3 s.t. sup

ρ∈R₊

max(0, g^′(ρ)) ρ^β′−+ρ^β+ <∞ and

∃1

3 ≤β₋^′ ≤β₊< 2

3 s.t. sup

(ρ,κ)∈R₊×R₊

max

0,∂h

∂ρ(ρ, κ)

ρ^β′−+ρ^β+ <∞.

As usual in the mathematical study of molecular electronic structure models, we embed (20) in the family of problems

I_λ = inf{E(γ), γ ∈ Kλ} (35)

parametrized by λ∈R₊ where Kλ=

γ ∈ S(L²(R³))|0≤γ ≤1, Tr (γ) =λ, Tr (−∆γ)<∞ , and introduce the problem at infinity

I_λ^∞= inf{E^∞(γ), γ∈ Kλ} (36)

where

E^KS(γ) = Tr (−∆γ) +J(ρ_γ) +E_xc(ρ_γ).

The following results hold true for both the LDA and GGA extended Kohn-Sham models.

Lemma 1. Consider (35) and (36) withE_xc given either by (21) or by (22) together with the conditions (25)-(28) or (29)-(32). Then

1. I₀ =I₀^∞= 0 and for all λ >0, −∞< I_λ< I_λ^∞<0;

2. the functions λ7→I_λ and λ7→I_λ^∞ are continuous and decreasing;

3. for all 0< µ < λ,

I_λ ≤I_µ+I_λ−µ^∞ . (37)

Our main results are the following two theorems.

Theorem 1 (Extended KS-LDA model). Assume that Z ≥ N = 2N_p (neutral or positively charged system) and that the function g satisfies (25)-(28). Then the extended Kohn-Sham LDA model (35) with E_xc given by (21) has a minimizer γ₀. Besides, γ₀ satisfies the self-consistent field equation

γ₀ =χ_(−∞,ǫF)(H_ργ0) +δ (38)

for some ǫ_F≤0, where H_ργ

0 =−1

2∆ +V +ρ_γ0⋆|r|⁻¹+g^′(ργ0),

whereχ_(−∞,ǫF)is the characteristic function of the range(−∞, ǫ_F)and whereδ∈ S(L²(R³)) is such that 0≤δ ≤1 and Ran(δ) =Ker(H_ργ0 −ǫ_F).

Theorem 2 (Extended KS-GGA model for two electron systems). Assume that Z ≥N = 2N_p = 2 (neutral or positively charged system with two electrons) and that the function h satisfies (29)-(34). Then the extended Kohn-Sham GGA model (35) with E_xc given by (22) has a minimizer γ₀. Besides, γ₀ = |φihφ| where φ is a minimizer of the standard spin-unpolarized Kohn-Sham problem (19) forN_p = 1, hence satisfying the Euler equation

−1 2div

1 + ∂h

∂κ(ρ_φ,|∇φ|²)

∇φ

+

V +ρ_φ⋆|r|⁻¹+∂h

∂ρ(ρ_φ,|∇φ|²)

φ=ǫφ (39) for some ǫ < 0, where ρ_φ = 2φ². In addition, φ ∈ C^0,α(R³) for some 0 < α < 1 and decays exponentially fast at infinity. Lastly, φcan be chosen non-negative and (ǫ, φ) is the lowest eigenpair of the self-adjoint operator

−1 2div

1 +∂h

∂κ(ρ_φ,|∇φ|²)

∇·

+V +ρ_φ⋆|r|⁻¹+∂h

∂ρ(ρ_φ,|∇φ|²).

We have not been able to extend the results of Theorem 2 to the general case of N_p electron pairs. This is mainly due to the fact that the Euler equations for (35) withE_xc given by (22) do not have a simple structure forN_p ≥2.

5 Proofs

For clarity, we will use the following notation E_xc^LDA(ρ) =

Z

R3

g(ρ(r))dr

E^GGA_xc (ρ) = Z

R3

h(ρ(r),1

2|∇√ρ(r)|²)dr E^LDA(γ) = Tr (−∆γ) +

Z

R3

ρ_γV +J(ρ_γ) + Z

R3

g(ρ_γ(r))dr E^GGA(γ) = Tr (−∆γ) +

Z

R3

ρ_γV +J(ρ_γ) + Z

R3

h(ρ_γ(r),1

2|∇√ρ_γ(r)|²)dr.

The notationsE_xc(ρ) and E(γ) will refer indifferently to the LDA or the GGA setting.

5.1 Preliminary results

Most of the results of this section are elementary, but we provide them for the sake of completeness. Let us denote by S₁ the vector space of trace-class operators on L²(R³) (see e.g. [25]) and introduce the vector space

H={γ ∈S₁ | |∇|γ|∇| ∈S₁}

endowed with the norm k · kH= Tr (| · |) + Tr (||∇| · |∇||), and the convex set K=

γ ∈ S(L²(R³))|0≤γ ≤1, Tr (γ)<∞, Tr (|∇|γ|∇|)<∞ . Lemma 2. For all γ ∈ K, √ρ_γ∈H¹(R³) and the following inequalities hold true

1

2k∇√ρ_γk²L² ≤Tr (−∆γ) (40)

0≤J(ργ)≤C(Trγ)³²(Tr (−∆γ))¹² (41)

−4Z(Trγ)¹²(Tr (−∆γ))¹² ≤ Z

R3

ρ_γV ≤0 (42)

−C

(Trγ)^1−β−2 (Tr (−∆γ))^3β−2 + (Trγ)^1−β2+(Tr (−∆γ))^3β2+

≤E_xc(ρ_γ)≤0(43) E(γ)≥ 1

2

(Tr (−∆γ))¹² −4Z(Trγ)¹²2

−8Z²Trγ

−C

(Trγ)

2−β−

2−3β− + (Trγ)

2−β+ 2−3β+

(44) E^∞(γ)≥ 1

2Tr (−∆γ)−C

(Trγ)

2−β−

2−3β− + (Trγ)

2−β+ 2−3β+

, (45)

for a positive constant C independent of γ. In particular, the minimizing sequences of (35) and those of (36) are bounded in H.

Proof. Anyγ ∈ K can be diagonalized in an orthonormal basis ofL²(R³) as follows γ =

+∞X

i=1

n_i|φihφ_i|

with n_i ∈ [0,1], φ_i ∈ H¹(R³), R

R3φ_iφ_j = δ_ij, Tr (γ) = P_+∞

i=1 n_i < ∞ and Tr (−∆γ) = P_+∞

i=1n_ik∇φ_ik²_L² <∞.As

|∇√ρ_γ|² = 2

+∞X

i=1

n_iφ_i∇φ_i

2

X+∞

i=1

n_iφ²_i ,

(40) is a straightforward consequence of Cauchy-Schwarz inequality. Using Hardy-Littlewood- Sobolev [16], interpolation, and Gagliardo-Nirenberg-Sobolev inequalities, we obtain

J(ρ_γ)≤C₁kρ_γk²

L⁶⁵ ≤C₁kρ_γk

3 2

L¹kρ_γk

1 2

L³ ≤C₂kρ_γk

3 2

L¹k∇√ρ_γkL².

Hence (41), using (40) and the relationkρ_γkL¹ = 2Tr (γ). It follows from Cauchy-Schwarz and Hardy inequalities and from the above estimates that

Z

R3

ρ_γ

| · −R_k|≤2kρ_γk

1 2

L¹k∇√ρ_γkL² ≤4(Trγ)¹²(Tr (−∆γ))¹².

Hence (42). Conditions (25)-(28) for LDA and (29)-(32) for GGA imply that E_xc(ρ) ≤0 and there exists 1< p₋< p₊< ⁵₃ (p_±= 1 +β_±) and some constantC ∈R₊ such that

∀ρ∈ K, |E_xc(ρ)| ≤C Z

R3

ρ^p−+ Z

R3

ρ^p+

, (46)

from which we deduce (43), using interpolation and Gagliardo-Nirenberg-Sobolev inequali- ties. Lastly, the estimates (44) and (45) are straightforward consequences of (41)-(43).

Lemma 3. Let λ >0 and γ ∈ Kλ. There exists a sequence (γ_n)_n∈N such that 1. for all n∈N, γ_n∈ Kλ, γ_n is finite-rank and Ran(γ_n)⊂C_c^∞(R³);

2. (γ_n)_n∈N converges toγ strongly in H;

3. (√ρ_γn)_n∈N converges to √ρ_γ strongly in H¹(R³);

4. (ρ_γn)_n∈N and(∇√ρ_γn)_n∈N converge almost everywhere toρ_γ and∇√ρ_γ respectively.

In particular

n→∞lim E(γ_n) =E(γ) and lim

n→∞E^∞(γ_n) =E^∞(γ). (47) Proof. Let γ∈ Kλ. It holds

γ = X+∞

i=1

n_i|φ_iihφ_i|

with n_i ∈ [0,1], φ_i ∈ H¹(R³), R

R3φ_iφ_j = δ_ij, Tr (γ) = P_+∞

i=1n_i = λ and Tr (−∆γ) = P_+∞

i=1n_ik∇φ_ik²_L² <∞.

We first prove that γ can be approached by a sequence of finite-rank operators. Let N₀ ∈ N such that 0< n_N0 < 1 (if no such N₀ exists, then γ is finite-rank and one can directly proceed to the second part of the proof). For allN ∈N, we set

e γ_N =

XN

i=1

n_i|φ_iihφ_i|+ λ− XN

i=1

n_i

!

|φ_N0ihφ_N0|.

For N large enough, eγ_N ∈ Kλ, and the sequence (eγ_N) obviously converges to γ in H. Besides, (ρ_eγN) converges a.e. to ρ_γ and

|ρ_eγN −ρ_γ| ≤ n_N0 +λ− XN

i=1

n_i

! φ²_N0 +

X+∞

i=N+1

n_i|φ_i|²≤ρ_γ+λφ²_N0.

Hence the convergence of (ρ_eγN) toρ_γ inL^p(R³) for all 1≤p≤3. Besides, for allN ≥N₀,

|∇√ρ_eγN|² = 2

XN

i=1, i6=N0

n_iφ_i∇φ_i+ n_N0+λ− XN

i=1

n_i

!

φ_N0∇φ_N0

2

XN

i=1, i6=N0

n_i|φ_i|²+ n_N0 +λ− XN

i=1

n_i

!

|φ_N0|²

≤2 X+∞

i=1

n_i|∇φ_i|²+2λ|∇φ_N0|².

Using Lebesgue dominated convergence theorem, we obtain that the sequence (k∇√ρ_eγNkL²) converges tok∇√ρ_γkL², from which we deduce that (√ρ_eγN) converges to√ρ_γ strongly in H¹(R³).

The second part of the proof consists in approaching each φ_i by a sequence of regular compactly supported functions. For each i, we consider a sequence (φ_i,k)_k∈^N of functions of C_c^∞(R³) such that

• supp(φ_i,k)⊂supp(φi) and R

R2φ_i,kφ_j,k =δ_ij for all k,

• (φ_i,k)_k∈N converges toφ_i strongly inH¹(R³) and almost everywhere,

• there existsh_i ∈L²(R³) such that|∇φ_i,k| ≤h_i for all k.

It is then easy to check that the sequence (eγ_N,k)_k∈N defined by e

γ_N,k= XN

i=1

n_i|φ_i,kihφ_i,k|+ λ− XN

i=1

n_i

!

|φ_N0,kihφ_N0,k|

converges toeγ_N inHand is such that (√ρ_eγN,k)_k∈Nconverges to√ρ_eγN strongly inH¹(R³).

One can then extract from (eγ_N,k)_(N,k)∈N^∗×Na subsequence (γ_n)_n∈Nwhich converges to γ inHand is such that (√ρ_γn)_n∈^N converges to√ρ_γ strongly inH¹(R³), and there is no restriction in assuming that (ρ_γn)_n∈N and (∇√ρ_γn)_n∈N converge almost everywhere toρ_γ and ∇√ρ_γ respectively.

The linear formγ 7→Tr (−∆γ) being continuous onHand the functionalsu7→R

R3u²V and u7→J(u²) +E_xc(u²) being continuous on H¹(R³), (47) holds true.

5.2 Proof of Lemma 1

Obviously,I₀ =I₀^∞= 0 andI_λ ≤I_λ^∞ for all λ∈R₊.

Let us first prove assertion 3. Let 0< µ < λ,ǫ >0 andγ ∈ Kµ such that I_µ≤ E(γ)≤ I_µ+ǫ. It follows from Lemma 3 that there is no restriction in choosing γ of the form

γ = XN

i=1

n_i|φ_iihφ_i|

with 0≤n_i ≤1,P_N

i=1n_i =µ,hφ_i|φ_ji=δ_ij and φ_i ∈C_c^∞(R³). Likewise, there exists γ^′ =

N^′

X

i=1

n^′_i|φ^′_iihφ^′_i|

with 0 ≤ n^′_i ≤ 1, P_N^′

i=1n^′_i = λ−µ, hφ^′_i|φ^′_ji = δ_ij and φ^′_i ∈ C_c^∞(R³), such that I_λ−µ^∞ ≤ E^∞(γ^′)≤I_λ−µ^∞ +ǫ. Letebe a unit vector ofR³andτ_athe translation operator onL²(R³) defined byτ_af =f(· −a) for allf ∈L²(R³). Forn∈N, we define

γ_n=γ+τ_neγ^′τ_−ne. It is easy to check that for nlarge enough, γ_n∈ Kλ and

I_λ ≤ E(γ_n)≤ E(γ) +E^∞(γ) +D(ρ_γ, τ_neρ_γ^′)≤I_µ+I_λ−µ^∞ + 3ǫ, where

D(ρ, ρ^′) :=

Z

R3

Z

R3

ρ(r)ρ^′(r^′)

|r−r^′| drdr^′. Hence (37).

Making use of similar arguments, it can also be proved that

I_λ^∞≤I_µ^∞+I_λ−µ^∞ . (48)

Let us now consider a function φ∈C_c^∞(R³) such that kφkL² = 1. For all σ > 0 and all 0≤λ≤1, the density operator γ_σ,λ with density matrix

γ_σ,λ(r,r^′) =λσ³φ(σr)φ(σr^′)

is inKλ. Using (28) for LDA and (32) for GGA, we obtain that there exists 1≤α < ³₂, c >0 andσ₀>0 such that for all 0≤λ≤1 and all 0≤σ≤σ₀,

I_λ^∞≤ E^∞(γ_σ,λ)≤λσ² Z

R3|∇φ|²+λ²σJ(2|φ|²)−cλ^ασ^3(α−1) Z

R3|φ|^2α.

ThereforeI_λ^∞<0 forλpositive and small enough. It follows from (37) and (48) that the functionsλ7→I_λ and λ7→I_λ^∞are decreasing, and that for all λ >0,

−∞< I_λ≤I_λ^∞<0.

To proceed further, we need the following lemma.

Lemma 4. Let λ >0 and(γ_n)_n∈N be a minimizing sequence for (35). Then the sequence (ρ_γn)_n∈N cannot vanish, which means that

∃R >0 s.t. lim

n→∞ sup

x∈R3

Z

x+BR

ρ_γn >0.

The same holds true for the minimizing sequences of (36).

Proof. Let (γ_n)_n∈^N be a minimizing sequence for (35). By contradiction, assume that

∀R >0, lim

n→∞ sup

x∈R3

Z

x+BR

ρ_n= 0.

Let 1< p < ⁵₃. For ρ≥0 such that√ρ∈H¹(R³), it holds for allk∈Z³, Z

k+B1

ρ^p ≤ Z

k+B1

ρ

p−1Z

k+B1

ρ^2−1p 2−p

≤C_p Z

k+B1

ρ

p−1Z

k+B1

(ρ+|∇√ρ|²)

(where the constant C_p does not depend on k). We therefore obtain Z

R3

ρ^p ≤ X

k∈Z3

Z

k+B1

ρ^p

≤ C_p X

k ∈Z3

Z

k+B1

ρ

p−1Z

k+B1

(ρ+|∇√ρ|²)

≤ 8C_p

sup

x∈R3

Z

x+B1

ρ

p−1Z

R3

(ρ+ Z

R3|∇√ρ|²)

. Hence, for all γ ∈ K,

Z

R3

ρ^p_γ≤16C_p

sup

x∈R3

Z

x+B1

ρ_γ p−1

kγk²H.

As we know that any minimizing sequence of (35) is bounded in H, we deduce from the above inequality that for all 1< p < ⁵₃,

n→∞lim Z

R3

ρ^p_γn = 0.

In particular, it follows from (46) that

n→∞lim Z

R3

E_xc(ργn) = 0.

Let us now fix 1 < p < ³₂, ǫ > 0 and R > 0 such that |V| ≤ ǫλ⁻¹ on B_R^c. For n large enough, we have

Z

R3

ρ_γnV ≤

Z

BR

ρ_γn|V|+ Z

B^c_R

ρ_γn|V| ≤ Z

BR

|V|^{p′ 1}

p′ Z

BR

ρ^p_γn ¹_p

+ ǫ λ

Z

B^c_R

ρ_γn ≤2ǫ.

Therefore

n→∞lim Z

R3

ρ_γnV = 0.

As,

E(γ_n)≥ Z

R3

ρ_γnV +E_xc(ρ_γn),

we obtain thatI_λ ≥0. This is in contradiction with the previously proved result stating that I_λ <0. Hence (ρ_γn)_n∈N cannot vanish. The case of problem (36) is easier since the only non-positive term in the energy functional isE_xc(ρ).