# Simple model for the spherically- and system-averaged pair density: Results for two-electron atoms

**ABSTRACT** As shown by Overhauser and others, accurate pair densities for the uniform electron gas may be found by solving a two-electron scattering problem with an effective screened electron-electron repulsion. In this work we explore the extension of this approach to nonuniform systems, and we discuss its potential for density functional theory. For the spherically- and system-averaged pair density of two-electron atoms we obtain very accurate short-range properties, including, for nuclear charge $Z\ge 2$, ``on-top'' values (zero electron-electron distance) essentially indistinguishable from those coming from precise variational wavefunctions. By means of a nonlinear adiabatic connection that separates long- and short-range effects, we also obtain Kohn-Sham correlation energies whose error is less than 4 mHartree, again for $Z\ge 2$, and short-range-only correlation energies whose accuracy is one order of magnitude better. Comment: 9 pages, 6 figures (14 .eps files); revised version, to appear in Phys. Rev. A

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**ABSTRACT:**Attempts to generalize the density functional theory are summarized. A possible pair density functional theory is linked to the Overhauser parametrization of the electron- gas pair density. The importance of the cumulant partitioning is stressed and a modied Overhauser approach for the cumulant 2-body reduced density matrix, the contraction of which determines the 1-body reduced density matrix, is discussed. - SourceAvailable from: Kyung-Soo Yi[Show abstract] [Hide abstract]

**ABSTRACT:**We first exploit the spin symmetry relation fss̅ xc(ζ)=fs̅ sxc(−ζ) for the exact exchange correlation kernel fss̅ xc(ζ) in an inhomogeneous many-electron system with arbitrary spin polarization ζ. The physical condition required to satisfy the specific symmetry relation fss̅ xc(ζ)=fs̅ sxc(ζ) is derived and examined for simple ferromagnetic-nonmagnetic structure by taking the electrochemical potential into account. The condition is then applied to several composite systems useful in spintronics applications such as the magnetic system with net spin polarization.Physical Review B 09/2006; 74(11). · 3.66 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**We propose a model for the angle- and system-averaged exchange-correlation hole of a many-electron system. This hole analyzes the exchange-correlation energy into contributions of various distances u from an electron. The model is “reverse-engineered” (derived from and not used to derive a density functional). It satisfies known exact hole constraints, including ones that can only be satisfied by a meta-generalized gradient approximation or meta-GGA. It incorporates the exchange-correlation energy density of the Tao-Perdew-Staroverov-Scuseria (TPSS) nonempirical meta-GGA. The hole model is tested for atoms and applied to jellium surfaces. The Fourier transform (u→k) of the hole is needed for wave-vector interpolation of the jellium surface energy from an exact small-k or large-u asymptote. We find essentially the same surface energies (close to the uncorrected TPSS values) whether we apply the wave-vector interpolation correction to the local spin density approximation, the GGA or the meta-GGA. These and other considerations suggest that these surface energies are accurate. Moreover, we find that the uncorrected TPSS surface energies have a realistic wave-vector analysis. Our TPSS hole model can be used to build the hole model for a TPSS-based global hybrid functional, or for a hyper-GGA that uses full exact exchange.Physical Review B 01/2006; 73(20). · 3.66 Impact Factor

Page 1

arXiv:cond-mat/0411179v2 [cond-mat.mtrl-sci] 8 Feb 2005

Simple model for the spherically- and system-averaged pair density:

Results for two-electron atoms

Paola Gori-Giorgi and Andreas Savin

Laboratoire de Chimie Th´ eorique, CNRS, Universit´ e Pierre et Marie Curie, 4 Place Jussieu, F-75252 Paris, France

(Dated: February 2, 2008)

As shown by Overhauser and others, accurate pair densities for the uniform electron gas may

be found by solving a two-electron scattering problem with an effective screened electron-electron

repulsion. In this work we explore the extension of this approach to nonuniform systems, and we

discuss its potential for density functional theory. For the spherically- and system-averaged pair

density of two-electron atoms we obtain very accurate short-range properties, including, for nuclear

charge Z ≥ 2, “on-top” values (zero electron-electron distance) essentially indistinguishable from

those coming from precise variational wavefunctions. By means of a nonlinear adiabatic connection

that separates long- and short-range effects, we also obtain Kohn-Sham correlation energies whose

error is less than 4 mHartree, again for Z ≥ 2, and short-range-only correlation energies whose

accuracy is one order of magnitude better.

I. INTRODUCTION AND SUMMARY OF

RESULTS

Density Functional Theory (DFT) [1, 2, 3] is nowa-

days the most widely used method for electronic struc-

ture calculations, in both condensed matter physics and

quantum chemistry, thanks to a combination of low com-

putational cost and reasonable accuracy.

In the application of this theory within the Kohn-Sham

(KS) formalism [4], one deals with a model system (the

KS system) of N noninteracting electrons in a local po-

tential vKS(r) that forces them to yield the same density

n(r) of the physical system. The energy of the physi-

cal system is then obtained from that of the KS system

via a functional of the density, whose only term not ex-

plicitly known is the exchange-correlation energy Exc[n].

Correspondingly, in the local potential vKS(r) there is an

unknown term, vxc(r) = δExc[n]/δn(r).

The success of KS DFT is mostly due to the fact that

even simple physical approximations of Exc[n], like the

local density approximation (LDA) [4], already give ac-

ceptable results for many purposes. This spurred fun-

damental research in the field, and led to a wealth of

more and more sophisticated exchange-correlation func-

tionals [2, 3, 5], and to the development of different ap-

proaches to DFT [6, 7].

Recently, in the search for accurate Exc[n], the fo-

cus of a large part of the DFT community has shifted

from seeking explicit functionals of the density like the

generalized gradient approximations (GGA) [8], to im-

plicit functionals, tipically using the Kohn-Sham orbital

kinetic energy density [9] or the Kohn-Sham orbitals

(see, e.g., [3, 10, 11]). The so-called “third generation”

of exchange-correlation functionals is based on the ex-

act exchange of the noninteracting (KS) system, sim-

ply obtained by putting in the formal expression for the

Hartee-Fock exchange the Kohn-Sham orbitals ϕiσ(r).

Such expression corresponds to an implicit functional

of the density, Ex[n] = Ex[{ϕiσ[n]}]. The local poten-

tial vx(r) = δEx[n]/δn(r) that generates the orbitals

ϕiσ(r)[n] can be obtained via the optimized effective po-

tential method (OEP) [12].

In this broad context, sketchily summarized here, we

propose a simplified method to build the “bridge” be-

tween the physical and the KS system, or, more gener-

ally, with a reference model system of partially interact-

ing electrons. We focus on a quantity which is known to

play a crucial role in DFT and has an intuitive physical

meaning, the spherically and system-averaged electronic

pair density f(r12) (also known in chemistry as spherical

average of the intracule density, see e.g. [13, 14, 15, 16],

and especially [17, 18]). Given the spin-resolved diagonal

of the two-body reduced density matrix,

γ(2)

σ1σ2(r1,r2) =

?

σ3...σN

?

|Ψ(r1σ1,...,rNσN)|2dr3...drN,

(1)

we define the spin-summed pair density n2(r1,r2),

n2(r1,r2) =N(N − 1)

2

?

σ1σ2

γ(2)

σ1σ2(r1,r2), (2)

and we integrate it over all variables but r12= |r2− r1|

by switching, e.g., to center-of-mass coordinates, R =

1

2(r1+ r2), r12= r2− r1,

?

4π

f(r12) =dRdΩr12

n2

?

R −r12

2,R +r12

2

?

.(3)

The function f(r12) times the volume element 4πr2

is proportional to the probability density for the particle-

particle distance in a system of N electrons in the state

Ψ, and is normalized to the number of electron pairs,

N(N − 1)/2 .

pectation value of the electronic Coulomb repulsion (in

Hartree atomic units used throughout),

12dr12

This quantity fully determines the ex-

?Vee? ≡ ?Ψ|Vee|Ψ? =

?∞

0

4πr2

12

f(r12)

r12

dr12,(4)

and is a measurable quantity, being essentially the

Fourier transform of the electronic static structure fac-

tor [19]. By construction, the one-electron density n(r) is

Page 2

2

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0 1 2 3

r12

4 5 6

f(r12)

’exact’

this work

Kohn-Sham

H-

0.04

0.08

0.12

0.16

0 0.5 1 1.5 2 2.5

f(r12)

r12

’exact’

this work

Kohn-Sham

He

0

0.2

0.4

0.6

0.8

0 0.2 0.4 0.6 0.8 1 1.2

f(r12)

r12

’exact’

this work

Kohn-Sham

Li+

0

0.5

1

1.5

2

0 0.2 0.4 0.6 0.8 1

f(r12)

r12

’exact’

this work

Kohn-Sham

Be2+

FIG. 1: Spherically and system-averaged pair densities for two-electron atoms: ’exact’ results [40] are compared with the values

obtained for the Kohn-Sham system and with the present approach, which is designed to get realistic f(r12) starting from the

Kohn-Sham ones.

the same in the KS and in the physical system, whereas

f(r12) will be different in the two cases, as shown, e.g., in

Fig. 1 for some two-electron atoms. In the physical sys-

tem f(r12) has a much lower “on-top” value f(r12= 0)

than in the KS system, and it has a cusp [20], as expected

from the fact that the electrons repel each other via the

Coulomb interaction. Roughly speaking, in the classic

DFT approach to correlation, the difference in energy

arising when we evaluate the r.h.s. of Eq. (4) with the

two f(r12), the physical and the KS, is what one tries to

describe with a universal functional of the density [21].

Here we follow a different approach: we try to build re-

alistic f(r12) from a set of simple radial equations, to be

solved for each system, and eventually coupled to a DFT

calculation.

Our approach is inspired by the seminal work of Over-

hauser [22] and its subsequent extension [23], in which the

function f(r12) for the uniform electron gas is obtained

from a set of geminals, solutions of a radial Schr¨ odinger

equation with an effective electron-electron (e-e) poten-

tial.Simple approximations for such effective e-e po-

tential give indeed accurate results at all relevant densi-

ties [23, 24, 25]. Here we try to generalize this approach

to systems of nonuniform density to get accurate f(r12).

The main goal of the present work is understanding

whether the method is promising, and whether it is worth

developing and refining it. To this purpose, we define the

formalism (Sec. II), we give a physically-motivated pre-

scription for the effective e-e potential (Sec. III), and we

test it on the simple but not trivial case of two-electron

atoms (Sec. IV). The prescription for the effective e-e

potential used here is not very sophisticated. Improve-

ments along the lines of what has been done for the uni-

form electron gas [23, 24, 25] will be the subject of future

work. Yet, even at this simple first stage of the theory

we already obtain rather accurate results, especially for

the short-range part of f(r12) (see Fig. 1 and Table I).

In Sec. V we show that with the present approach we can

also recover the difference in kinetic energy between the

physical and the KS system. Finally, Sec. VI is devoted

to conclusions, perspectives and open questions.

II.FORMALISM

In addition to the work on the “Overhausermodel” [22,

23, 24], the approach described here takes advantage of

inspiring papers on the possibility of constructing a pair-

density functional theory [26, 27, 28, 29], and a local-

density-of-states functional theory [30].

Our starting point is a constrained search over1

1) “effective” orthonormal geminals ψi(r12) that mini-

mize the electron-electron relative kinetic energy T12=

−∇2

2N(N−

r12(the reduced mass for the relative motion is 1/2)

Page 3

3

and yield the exact f,?

i|ψi(r12)|2= f(r12),

?

i

min

{ψi}→f

?ψi| − ∇2

r12|ψi?, (5)

thus leading to a set of radial equations formally similar

to the KS ones,

[−∇2

N(N−1)/2

?

i=1

r12+ veff(r12)]ψi(r12) = ǫiψi(r12) (6)

|ψi(r12)|2= f(r12). (7)

These equations imply that an expansion in spherical

harmonics of f(r12) has been done, so that the kinetic

energy operator also contains the usual ℓ(ℓ+1)/r2term.

To fully define these equations we need a rule for the oc-

cupancy of the effective geminals. In analogy with what

has been done for the uniform electron gas [23, 24], we

can assign spin degeneracy 1 to even-angular-momentum

states (singlet) and spin degeneracy 3 to odd-angular-

momentum states (triplet), up to N(N − 1)/2 occupied

states. More generally, for open shell systems it could

be better to develop the formalism for the spin-resolved

quantities, starting from Eq. (1). This will be investi-

gated in future work.

The effective electron-electron potential veff(r12) of

Eq. (6) is the Lagrange parameter for f(r12), and is a

functional of f itself and of the electron-nucleus external

potential Vne (or, equivalently, of the density n(r)). To

see this, we can rewrite our Eqs. (6)-(7) in terms of a

minimization of the total energy in two steps, using the

constrained search formalism [31, 32] for the ground state

energy E = minΨ?Ψ|T + Vee+ Vne|Ψ?,

E = min

f

min

Ψ→f

?

min

{ψi}→f

?

i

?ψi| − ∇2

r12|ψi? +

?

f

r12dr12

+?Ψ|T + Vne|Ψ? − min

{ψi}→f

?

i

?ψi| − ∇2

r12|ψi?

?

.(8)

Defining the kinetic and external-potential functional as

FKE[f;Vne] =

?ψi| − ∇2

min

Ψ→f?Ψ|T + Vne|Ψ? − min

{ψi}→f

?

i

r12|ψi?,(9)

we can rewrite

E = min

f

?

min

{ψi}→f

?

i

?

?ψi| − ∇2

r12|ψi? +

?

f

r12dr12

+FKE[f;Vne]. (10)

Searching this minimum by directly varying the ψi(with

given, fixed, Vne) leads to Eqs. (6)-(7) with the identifi-

cation

veff(r12) =

1

r12

+δFKE[f;Vne]

δf(r12)

. (11)

Thus, in principle we could recover the whole ground-

state energy via the (unknown) system-dependent func-

tional FKE[f;Vne]. In practice, it seems much more fea-

sible to combine Eqs. (6)-(7) with a DFT calculation,

that yields the complementary information (the density,

and thus ?Ψ|Vne|Ψ?). The steps of Eqs. (8)-(11) can be

repeated for arbitrary electron-electron interaction and

external one-body potential. In particular, we can set

Vλ

tential that keeps the density equal to the one of the

physical system. One could thus obtain fλat each cou-

pling strength λ between 0 and 1 from Eqs. (6)-(7) with

a suitable vλ

eff. The correlation energy of KS theory is

then simply given by [6, 33, 34]

ee= λVeeand Vne= Vλ, where Vλis an external po-

Ec[n] =

?1

0

dλ

?

dr12fλ(r12) − fλ=0(r12)

r12

. (12)

Alternatively, this procedure (usually called adiabatic

connection [34]) can be performed along a nonlinear path,

e.g., by setting [6, 35, 36, 37] vλ

erf(x) is the error function (see Sec. V). Eventually, the

two sets of equations, KS and (6)-(7) plus (12), could

be solved together self-consistently. This last issue is dis-

cussed in Sec. VI. Notice that if we combine Eqs. (6), (7)

and (12) with a DFT calculation, we only need to approx-

imate the potential vλ

eff(r12) and not the whole functional

FKEsince the remaining information is provided by DFT.

It is also worth to stress at this point that there is no

wavefunction behind our Eqs. (6)-(7): the effective gem-

inals ψiare defined via Eq. (5), and by specifying their

occupancy (e.g., triplet and singlet). A bosonic version

of the theory, in which only one geminal (proportional

to

?f(r12)) is occupied can also be considered [29, 38].

In this work we only focus on two-electron systems for

which the two choices are equivalent. A careful com-

parison of performances of the “fermion-like” and of the

“boson-like” occupancy in the uniform electron gas is the

subject of current investigations [39].

As for KS DFT, the formalism just described can be

useful only if simple approximations for veff(r12) yield ac-

curate results. This is what we start to check in the rest

of this paper. First, we construct a physically-motivated

veff for two-electron atoms for the fully-interacting sys-

tem, and we compare our results with “exact” ones [40].

Then, we generalize our construction to build veff along

the adiabatic connection, and we calculate the KS corre-

lation energy.

ee= erf(λr)/r, where

III.

POTENTIAL: THE OVERHAUSER MODEL

EFFECTIVE ELECTRON-ELECTRON

For the interacting electron gas of uniform density n,

Overhauser [22] proposed a simple and reasonable effec-

tive potential veff(r12): he took the sphere of volume n−1

around a given electron as the boundary within which

the other electrons are excluded, due to exchange and

Page 4

4

correlation effects. In the standard uniform-electron-gas

model, a rigid positively-charged background maintains

the electrical neutrality. Thus the exclusion region (or

“hole”) around a given electron, modeled with a sphere

of radius rs= (4πn/3)−1/3, uncovers the background of

positive charge, leading to an effective screened Coulomb

potential with screening length rs,

vOv

eff(r12;rs) =

1

r12

−

?

|r|≤rs

n

|r − r12|dr, (13)

equal to

vOv

eff(r12;rs) =

vOv

eff(r12;rs) =

1

r12+

r2

2r3

12

s−

0

3

2rs

r12≤ rs

r12> rs. (14)

Equations (6)-(7), combined with the Overhauser effec-

tive potential of Eq. (14) gave extremely accurate results

for the short-range part (r12≤ rs) of the function f(r12)

in the uniform electron gas at all relevant densities [23].

A more sophisticated effective potential, based on a self-

consistent Hartree approximation, extended such accu-

racy to the long-range part of f(r12) at metallic den-

sities [24]. Other approximate veff(r12) for the uniform

electron gas have also been proposed [25], and exact prop-

erties have been derived [41].

To produce realistic f(r12) for nonuniform systems

from Eqs. (6)-(7), here we generalize the original idea

of Overhauser [22, 23] to two-electron atoms, and show

that it gives rather accurate results, especially for the

short-rangepart of f(r12). We start from the effective po-

tential v(0)

eff(r12) that generates fKS(r12), the spherically-

and system averaged pair density of the Kohn-Sham sys-

tem.In the special case of a spin-compensated two-

electron system, the KS wavefunction is simply equal to

1

2

?n(r1)?n(r2). Because, at this first stage, we are in-

terested in testing our method as a “bridge” between

the KS and the real system, here we use the “exact”

Kohn-Sham system. We thus take accurate one-electron

densities [40], and construct fKS(r12),

fKS(r12) =1

4

?

n

?

R −r12

2

?

n

?

R +r12

2

?

dRdΩr12

4π

,

(15)

and the corresponding “exact” potential v(0)

can be calculated by inverting Eqs. (6)-(7),

eff(r12), that

v(0)

eff=∇2√fKS

√fKS

+ const.(16)

For systems with more than two electrons, the poten-

tial v(0)

effcould be calculated, e.g., with the methods of

Refs. [42, 43]. In practice, it would be much more efficient

to build approximations also for v(0)

amples of functions fKSfor nuclear charges Z = 1,2,3,4

are given in Fig. 1: they have a maximum at r12 = 0,

as expected in a system of two non-interacting electrons

with antiparallel spins in a confining one-body external

eff(see Sec. VI). Ex-

H−

2.1

He

0.86

Li+

0.54

Be2+

0.39

Ne8+

0.15rs

f(0)

“exact”

LDA

0.0021 0.104 0.528 1.526

0.0027 0.106 0.534 1.523

0.0047 0.119 0.563 1.587

32.6

32.7

33.0

rmax

12

“exact”

0.835

0.927

0.193 0.083 0.0465 0.0074

0.194 0.083 0.0465 0.0074

f(rmax

“exact”

12 )0.0031 0.114 0.55

0.0040 0.117 0.56

1.56

1.56

32.74

32.74

?Vee? − ?Vee?KS -0.12

“exact”

-0.097 -0.10

-0.078 -0.082 -0.089 -0.09

-0.10-0.10

-0.07

TABLE I: Our results for the function f(r12) for two-electron

atoms (first line for each property) compared with the corre-

sponding “exact” quantities [40]. In the first line of the table

we report the average rsas defined by Eqs. (17) and (19). For

the “on-top” value f(0) we also show the LDA result (with

f(0) for the uniform electron gas from Ref. [23]). All values

are in Hartree atomic units.

potential. When the interaction is turned on, the aver-

age distance between the two electrons increases, with

the constraint that n(r) is kept fixed. We can thus imag-

ine that, with respect to the Kohn-Sham system, in the

physical system the Coulomb repulsion between the elec-

trons creates, on average, a screening “hole” around the

reference electron of volume (n)−1, where n is an average

density (i.e., n(r) integrated over the wavefunction),

n =

1

N

?

drn(r)2.(17)

An approximate veff(r12) could thus be simply con-

structed as

veff(r12) ≈ v(0)

eff(r12) + vOv

eff(r12;rs) (18)

with an average rsin vOv

effof Eq. (14),

rs=?4π

3n?−1/3.(19)

The Overhauser-like potential vOv

relation potential to be added to the one that generates

fKS. It describes the correlation between pairs of elec-

trons due to Coulomb interaction, and keeps the informa-

tion on the one-electron density in an approximate way,

via the average n of Eq. (17). Of course, for more com-

plicated systems we expect to need a more sophisticated

construction for rs.

eff(r12;rs) is thus a cor-

IV.RESULTS

We have inserted the potential of Eq. (18) into Eqs. (6)-

(7), and solved them for several two-electron atoms. Our

results are shown in Fig. 1 and summarized in Table I.

Page 5

5

We see that the simple effective potential of Eq. (18) gives

already reasonable results for Z = 1 and 2, and that the

accuracy of the results increases with Z (as the system

becomes less and less correlated). The “on-top” value

f(0) is essentially exact for Z ≥ 2, and is much better

than the LDA estimate (normally regarded as accurate)

for all Z. This feature is appealing, since the on-top value

plays an important role in DFT [44], and accurate f(0)

are not easy to obtain from ab initio methods (see, e.g.,

Ref. [45] and references therein). The term 1/r12in the

effective potential ensures that the calculated f(r12) sat-

isfies the exact cusp condition f′(0) = f(0). Table I also

shows that the position rmax

12

the maximum of f is very well predicted by the present

approach. The presence of this maximum is essentially

due to the combined effect of the Coulomb repulsion be-

tween the electrons and the confining external potential.

In Fig. 2 we consider He and Ne8+, and we compare

the correlated part of our f, fc = f − fKS, with the

“exact” result [40] and with the corresponding quantity

calculated within LDA, i.e.,

and the height f(rmax

12 ) of

fLDA

c

(r12) =1

2

?

n(r)2gc(r12;n(r))dr,(20)

where gc is the pair-correlation function of the uni-

form electron gas at full coupling strength, taken from

Ref. [46]. (For an extended system of uniform density n,

we have gc= 2fc/nN.) Figure 3 shows the same quanti-

ties multiplied by 4πr12, i.e. the integrand of Eq. (4) for

the correlation part of ?Vee?: the area under each curve

gives ?Vee?−?Vee?KS. In the last line of Table I we report

quantitative results for ?Vee?−?Vee?KS. This quantity is

less accurate than the short-range properties, but it is

still encouraging. Moreover, it saturates for large Z as in

the exact case.

V.ADIABATIC CONNECTION AND

CORRELATION ENERGY

For the calculation of the energy of the physical sys-

tem, in addition to Vc[n] = ?Vee?−?Vee?KS, one needs to

know the kinetic-energy difference, Tc[n] = ?T?−?T?KS,

that can be obtained via the adiabatic connection for-

malism [6, 33, 34]. By varying a parameter λ, the in-

teraction vλ

ee(r12) between the electrons is switched on

continuously from zero to 1/r12, while the density is

kept fixed by an external one-body potential Vλ.

vλ=0

ee

= 0 and vλ=a

ee

= 1/r12, the KS correlation energy

Ec[n] = Tc[n] + Vc[n] is given by [6, 34]

If

Ec[n] =

?a

0

dλ

?∞

0

dr124π r2

12fλ

c(r12)∂vλ

ee(r12)

∂λ

, (21)

where fλ

Usually, the adiabatic connection is performed along a

linear “path” [11, 33], by setting vλ

to Eq. (12). If one is able to compute the exact fλ

c= fλ− fKS.

ee= λ/r12, which leads

c, the

-0.08

-0.06

-0.04

-0.02

0

0 0.5 1 1.5 2 2.5

fc(r12)

r12

He

’exact’

this work

LDA

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

0 0.1 0.2 0.3 0.4 0.5

fc(r12)

r12

Ne8+

’exact’

this work

LDA

FIG. 2: The correlated part of the spherically- and system-

averaged pair density, fc(r12) = f(r12)−fKS(r12). Our results

for He and Ne8+are compared with the “exact” ones and with

the LDA result (the hole for the uniform electron gas is taken

from Ref. [46]).

resulting Ecfrom Eq. (21) is independent of the choice

of vλ

ee. However, when approximations are made some

“paths” can give much better results than others [6]. As

we shall see, this is the case with the present approach.

We build an Overhauser-like potential for interaction

vλ

eff) as

ee(to be added to v(0)

vOv, λ

eff

(r12;rs) = vλ

ee(r12)−

?

|r|≤rs

nvλ

ee(|r−r12|)dr. (22)

That is, the average density n of Eq. (17) (and thus

the average rs) is kept fixed to mimic the fact that the

one-electron density does not change along the adiabatic

connection. The modified interaction vλ

a sphere of radius rs and of positive uniform charge of

density n that attracts the electrons with the same mod-

ified interaction.This attractive background approxi-

mates the effect of the external potential Vλon f.

eeis screened by

A.Linear adiabatic connection

If we choose vλ

vOv, λ

eff

(r12;rs) = λvOv

given by Eq. (14).

The results for ?Vλ

shown in Fig. 4, and are compared with the “exact”

ones of Ref. [42].The correlation energy Ec can be

ee

=λ/r12

we simply obtain

eff(r12;rs) is

eff(r12;rs), where vOv

ee? − ?Vee?KS for He and Ne8+are

Page 6

6

-0.016

-0.012

-0.008

-0.004

0

0.004

0 0.5 1 1.5 2 2.5

4 π fc(r12) r12

r12

He

’exact’

this work

LDA

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0 0.1 0.2 0.3 0.4 0.5 0.6

r12

4 π fc(r12) r12

Ne8+

’exact’

this work

LDA

FIG. 3: The real space analysis of the correlation part of

the expectation value of Vee: the area under each curve gives

?Vee? − ?Vee?KS (see also Eq. (4) and Fig. 2). Our results for

He and Ne8+are compared with the “exact” ones and with

the LDA result (the hole for the uniform electron gas is taken

from Ref. [46]).

calculated as the area under each curve.

Ec = −0.052 Hartree for He and Ec = −0.053 Hartree

for Ne8+, to be compared with the corresponding ’exact’

results, -0.042 and -0.045, respectively.

We obtain

B.A nonlinear adiabatic connection

As shown by Figs. 1–3 and Table I, the Overhauser-like

potential gives accurate results for the short-range part of

fc(r12). We can thus expect to obtain better correlation

energies from the adiabatic connection formalism if we

choose a modified interaction vλ

long-range and short-range contributions, like the “erf”

interaction [6, 35, 36, 37]

eethat is able to separate

vλ

ee(r12) =erf(λr12)

r12

.(23)

With this choice, Eq. (21) becomes

Ec[n] =

?∞

0

dλ

?∞

0

dr124π r2

12fλ

c(r12)

2

√πe−λ2r2

12. (24)

For large λ, when we are approaching the physical sys-

tem, the gaussian factor e−λ2r2

long-range contribution of fλ

cto the energy integrand. At

the KS end of the adiabatic connection, when λ → 0, the

12in Eq. (24) quenches the

-0.1

-0.08

-0.06

-0.04

-0.02

0

0 0.2 0.4 0.6 0.8 1

〈Vλ

ee〉 − 〈Vee〉KS

λ

He

’exact’

this work

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

0

0 0.2 0.4 0.6 0.8 1

〈Vλ

ee〉 − 〈Vee〉KS

λ

Ne8+

’exact’

this work

FIG. 4: Correlation part of ?Vλ

connection for He and Ne8+. Our results are compared with

the “exact” ones of Ref. [42]. The area under each curve gives

the correlation energy Ec of standard Kohn-Sham theory.

ee? along the linear adiabatic

interaction, and thus fλ

tribution to Eccoming from λ-values for which the long-

range part of fλ

cis not quenched is moderate. Moreover,

the function fλ

cis correctly normalized to zero so that for

λ → 0, not only is fλ

vanishes. In the linear adiabatic connection of Eq. (12),

instead, the long-range part of fλ

role in the energy integrand at all λ. Indeed, with this

nonlinear adiabatic connection we obtain Ec= −0.0405

Hartree for He and Ec= −0.0413 for Ne8+, much closer

to the “exact” values with respect to the results from the

linear adiabatic connection.

The technical details of this calculation are as fol-

lows.The potential vOv, λ

eff

be computed analytically, and is reported in Ap-

pendix A. We thus obtained, via Eqs. (6)-(7), dEλ

?dr12fλ

20 for He, and between 0 and 100 for Ne8+. We then fit-

ted our results with the derivative of the following func-

tional form

c, become small, so that the con-

csmall, but also the integral itself

cplays an important

(r12;rs) of Eq. (22) can

c/dλ =

c(r12)

2

√πe−λ2r2

12for 23 values of λ between 0 and

Eλ

c= −a1x6+ a2x8+ a3x10

(1 + b2x2)5

,x =λ

Z,

(25)

that has exact asymptotic behaviors for small and large

λ [37].(We have numerical evidence that our results

fulfill such exact behaviors.)

numerical values for dEλ

c/dλ, together with the derivative

of the fitting function of Eq. (25). The parameters and

In Fig. 5 we report our

Page 7

7

-0.014

-0.012

-0.01

-0.008

-0.006

-0.004

-0.002

0

0 5 10

λ

15 20

dEλ

c / dλ

He

fit

results

-0.0025

-0.002

-0.0015

-0.001

-0.0005

0

0 20 40 60 80 100

dEλ

c / dλ

λ

Ne8+

fit

results

FIG. 5: The derivative dEλ

connection defined by Eqs. (23)–(24) for He and Ne8+. Our

results are compared with the derivative of the fitting function

of Eq. (25). The area under each curve from zero to ∞ gives

the correlation energy Ec of standard Kohn-Sham theory.

c/dλ along the nonlinear adiabatic

the r.m.s of residuals are reported in Table II. The KS

correlation energy Ec[n] is then given by a3/b10.

The accuracy of our results with the “erf” adiabatic

connection is of particular interest for the method of

Refs. [6, 35, 36, 37], which combines multideterminantal

wavefunctions (configuration interaction, CI) with den-

sity functional theory (“CI+DFT”). In such approach,

instead of the KS system, one choses a reference system

of partially interacting particles, usually with the poten-

tial of Eq. (23). This model system is treated with a

multideterminantal wavefunction, in a CI fashion, that

allows to treat near-degeneracy effects. The remaining

part of the energy is calculated via a density functional,

that needs to be approximated. The larger λ, the larger

is the energy fraction treated with the CI calculation,

and thus the larger is the computational cost. The corre-

lation energy functional that needs to be approximated

is [6, 35, 36, 37]

E

λ

c[n] ≡ Ec[n] − Eλ

c[n], (26)

and can be rewritten as

?∞

E

λ

c[n] =

λ

dλ′

?∞

0

dr124π r2

12fλ′

c(r12)

2

√πe−λ′2r2

12.

(27)

Thus, only the short-range part of fλ

functional E

c[n], and we expect to get accurate results

with the present approach. Indeed, this is the case, as

ccontributes to the

λ

a1

a2

a3

b r.m.s.

He1.2047 2.3253 2.7788 1.5263 4·10−5

Ne8+0.3983 0.4711 0.4026 1.2557 9·10−6

TABLE II: Optimal fit parameters and r.m.s of the residuals

for the derivative of Eq. (25), that parametrizes our results

for dEλ

by Eqs. (23)–(24). See also Fig. 5.

c/dλ along the nonlinear adiabatic connection defined

-0.05

-0.04

-0.03

-0.02

-0.01

0

0 2 4 6 8 10

Ec − Ec

λ

λ

He

’exact’

this work

-0.05

-0.04

-0.03

-0.02

-0.01

0

0 10 20 30 40 50 60 70 80

λ

Ec − Ec

λ

Ne8+

’exact’

this work

FIG. 6: A nonlinear adiabatic connection that separates long-

and short-range effects: difference between the correlation en-

ergy Ec of the physical system (with full interaction 1/r) and

the correlation energy Eλ

tion erf(λr)/r, for He and Ne8+. Our results are compared

with the “exact” ones of Ref. [36].

c of the system with partial interac-

shown in Fig. 6, where we compare our results as a func-

tion of λ with the “exact” ones of Ref. [36]. The error

is less than 0.5 mHartree for λ ? 1/rs, that is a very

reasonable choice for the value of λ to be used in the

CI+DFT method of Refs. [6, 35, 36, 37].

VI. CONCLUSIONS AND PERSPECTIVES

In this work we have started to explore the possibil-

ity of solving simple radial equations to generate realistic

spherically- and system-averaged electronic pair densities

f(r12) for nonuniform systems. With a simple approxi-

mation for the unknown effective electron-electron inter-

action that appears in our formalism, we have obtained,

for two-electron atoms, results that are in fair agreement

with those coming from accurate variational wavefunc-

Page 8

8

tions (Figs. 1–3 and Table I). We have then extended

our approach along a nonlinear adiabatic connection and

obtained Kohn-Sham correlation energies whose error is

less than 4 mHartrees, and short-range-only correlation

energies whose accuracy is one order of magnitude better

(Fig. 6).

In Sec. II, we have introduced a general formalism for

many-electron systems that will be further tested in fu-

ture work. So far we can say that this formalism, com-

bined with simple physical approximations, works very

well for two completely different systems: the uniform

electron gas [23, 24, 25] and the He series. We think that

this fact makes the method promising.

To fully develop the approach described in this paper,

many steps have to be performed. First of all, the KS

part of the effective e-e potential, v(0)

should be also approximated, to make the extension to

many-electron systems practical. The correlation part of

the effective e-e potential can be improved, in analogy

with the recent developments for the uniform electron-

gas case [24, 25]. It should then be possible to construct

a self-consistent scheme (OEP-like) that combines the

Kohn-Sham equations with the correlation energy func-

tional arising from our approach [Eqs. (6)-(7) at different

coupling strengths λ, plus Eq. (12) or Eq. (24)]. With

respect to traditional DFT calculations, this combined

scheme would have the advantage of yielding not only

the ground-state one-electron density n(r) and energy E,

but also the spherically- and system-averaged pair den-

sity f(r12), thus allowing to calculate expectation values

eff(r12) of Eq. (18),

of two-body operators that only depend on the electron-

electron distance. The combination of our approach with

the CI+DFT method of Refs. [6, 35, 36, 37] could be also

implemented and, in view of the results of Fig. 6, it is

even more promising. We are presently working in all of

these main directions.

Acknowledgments

We thank C. Umrigar for the wavefunctions of

Ref. [40], J. Toulouse for the results of Ref. [36], E.K.U.

Gross, W. Kohn, M. Polini, G. Vignale and P. Ziesche

for encouraging discussions, V. Sahni for useful hints,

and J.K. Percus for helpful suggestions. This research

was supported by a Marie Curie Intra-European Fellow-

ships within the 6th European Community Framework

Programme (contract number MEIF-CT-2003-500026).

APPENDIX A: OVERHAUSER-LIKE

POTENTIAL FOR THE ERF INTERACTION

The evaluation of Eq. (22) with the interaction vλ

erf(λr12)/r12gives

ee=

vOv, λ

eff

(r12;rs) =u(s,µ)

rs

, (A1)

where s = r12/rs, µ = λrs, and

u(s,µ) =erf(µs)

s

−

1

8√π sµ3

?

2

?

1 +?−2 + s + s2?µ2+ e−4s µ2(−1 +?2 + s − s2?µ2)

erf [µ (1 − s)] +√π µ

?

e−(1−s)2µ2−

√π µ

?

3s + 2(1 − s)2(2 + s) µ2?

?

−3s + 2 (2 − s) (1 + s)2µ2?

erf [µ (1 + s)]

?

.(A2)

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