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1 Interpretation of the Kohn--Sham Electron--Interaction Energy Functional EeeKS [ρ] and Its Derivative vee(r)

# 1 Interpretation of the Kohn--Sham Electron--Interaction Energy Functional EeeKS [ρ] and Its Derivative vee(r)

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188

5 Physical Interpretation of Kohn–Sham Density Functional Theory …

with

F eff (r) = E ee (r) + Z tc (r).

(5.6)

KS

[ρ]/δρ(r) is that it is

Hence, the physical meaning of the functional derivative δ E ee

the work done to bring the model fermion from some reference point at infinity to its

position at r in the force of the conservative field F eff (r). Since ∇ × F eff (r) = 0, this

work done is path independent. Once again, the electron–interaction and Correlation–

Kinetic contributions to the functional derivative vee (r) are explicitly defined via

Q–DFT.

From the above interpretation of the potential energy vee (r) we have

∇vee (r) = ∇

KS

[ρ]

δ E ee

δρ(r)

= −F eff (r),

(5.7)

or, equivalently employing (5.1) and (5.6) that

δTc

δ E ee

+

δρ(r) δρ(r)

= −(E ee (r) + Z tc (r)).

(5.8)

This equation relates the functional derivatives of E ee and Tc to the component fields

E ee (r) and Z tc (r). Note, however, that

δ E ee

δρ(r)

= −E ee (r),

(5.9)

δTc

δρ(r)

= −Z tc (r).

(5.10)

and

These inequalities hold whether or not the fields E ee (r) and Z tc (r) are conservative.

The equality of the functional derivatives to the fields is that given by (5.5) or (5.8).

Since the pair–correlation density may also be written as g(rr ) = ρ(r ) + ρxc (rr ),

where ρxc (rr ) is the Fermi–Coulomb hole charge, the electron–interaction field

E ee (r) of (4.56) may be expressed as the sum of its Hartree E H (r) and Pauli–Coulomb

E xc (r) components:

(5.11)

E ee (r) = E H (r) + E xc (r),

where

E H (r) =

ρ(r )(r − r )

dr and E xc (r) =

|r − r |3

ρxc (rr )(r − r )

dr .

|r − r |3

(5.12)

5.1 Interpretation of the Kohn–Sham Electron–Interaction Energy Functional …

189

As the field E H (r) is due to a static or local charge distribution ρ(r), it may be

expressed as

(5.13)

E H (r) = −∇WH (r),

with the scalar potential energy WH (r) being

WH (r) =

ρ(r )

dr .

|r − r |

(5.14)

Thus, ∇ × E H (r) = 0. Equivalently, the potential energy WH (r) is the work done in

the conservative field E H (r):

WH (r) = −

r

E H (r ) · d .

(5.15)

A comparison of (4.91) and (5.14) shows that

vH (r) = WH (r).

(5.16)

Thus, the physical interpretation of the functional derivative δ E H [ρ]/δρ(r) is that

it is the work done to move a model fermion from its reference point at infinity to its

position at r in the force of the conservative field E H (r). Equivalently

δ E H [ρ]

δρ(r)

= −E H (r).

(5.17)

The Hartree energy functional E H [ρ] of (4.88), which is the energy of self–interaction

of the density, may also be expressed in terms of the Hartree field E H (r) as

EH =

ρ(r)r · E H (r)dr.

(5.18)

Again, employing the partitioning of the pair–correlation density g(rr ) into its

local and nonlocal components, we can write the quantum–mechanical electron–

interaction energy E ee as

E ee = E H + E xc ,

(5.19)

where E xc is the Pauli–Coulomb energy. Thus, the KS electron–interaction energy

functional (5.1) is

KS

= E H + E xc + Tc .

(5.20)

E ee

Comparison with (5.19) then defines the KS ‘exchange–correlation’ energy functional

in terms of the Pauli and Coulomb correlations and Correlation–Kinetic effects as

KS

[ρ] = E xc + Tc ,

E xc

(5.21)

190

5 Physical Interpretation of Kohn–Sham Density Functional Theory …

where E xc is expressed in terms of the Pauli–Coulomb field E xc (r) as

E xc =

ρ(r)r · E xc (r)dr,

(5.22)

and with Tc as previously defined by (5.3).

The KS ‘exchange–correlation’ potential energy vxc (r) is the work done to bring

the model fermion from a reference point at infinity to its position at r in the conservative field F xctc (r):

vxc (r) =

KS

[ρ]

δ E xc

=−

δρ(r)

r

F xctc (r ) · d ,

(5.23)

where

F xctc (r) = E xc (r) + Z tc (r).

(5.24)

This follows from (5.5) using the fact that the Hartree field E H (r) is conservative so

that ∇ × F xctc (r) = 0. Equivalently,

∇vxc (r) = ∇

KS

[ρ]

δ E xc

δρ(r)

= −(E xc (r) + Z tc (r)).

(5.25)

KS

[ρ] and its functional

Thus, the KS ‘exchange–correlation’ energy functional E xc

derivative vxc (r) can be expressed in terms of the Pauli–Coulomb E xc (r) and

KS

[ρ]

Correlation–Kinetic Z tc (r) fields. Hence, the dependence of the functional E xc

and its derivative vxc (r) on the separate electron correlations due to the Pauli principle, Coulomb repulsion, and Correlation–Kinetic effects is explicitly defined within

the framework of Q–DFT.

Substituting (5.21) into (5.25) leads to

δTc

δ E xc

+

δρ(r) δρ(r)

= −(E xc (r) + Z tc (r)),

(5.26)

which relates the functional derivative of the quantum–mechanical exchange–

correlation E xc and Correlation–Kinetic Tc energies to the fields E xc (r) and Z tc (r)

that give rise to them, respectively. Again, irrespective of whether the field E xc (r) is

conservative or not

δ E xc

= −E xc (r).

(5.27)

δρ(r)

Thus, we see that the mathematical entities of KS–DFT, viz. the electron-interaction

KS

KS

[ρ], its Hartree E H [ρ] and ‘exchange-correlation’ E xc

[ρ]

energy functional E ee

components, and their respective functional derivatives vee (r), v H (r), and vxc (r) can

all be afforded a rigorous physical interpretation.

5.1 Interpretation of the Kohn–Sham Electron–Interaction Energy Functional …

191

We next turn to the physical interpretation of the KS ‘exchange’ E xKS [ρ] and

‘correlation’ E KS

c [ρ] energy functionals, and of their respective derivatives vx (r) and

vc (r) in terms of the various electron correlations. This is achieved [15, 16] via

adiabatic coupling constant perturbation theory [17] as applied to both Q–DFT and

KS–DFT. The interpretations then follow on comparison of terms of equal order. We

begin by first describing the adiabatic coupling constant scheme, and Q–DFT and

KS–DFT within this framework.

5.2 Adiabatic Coupling Constant Scheme

In the adiabatic coupling constant (λ) scheme [5–7], the Hamiltonian Hˆ λ is defined

as

(5.28)

Hˆ λ = Tˆ + Vˆλ + λUˆ ; 0 ≤ λ ≤ 1,

where Tˆ and Uˆ are the usual kinetic and electron interaction operators and where the

external potential energy operator Vˆλ = i vλ (ri ). The corresponding Schrödinger

equation is

(5.29)

H λ ψ λ (X) = E λ ψ λ (X),

where ψ λ (X) is the ground state wavefunction for interaction strength λ. The real

interacting system corresponds to λ = 1. In (5.28), the operator Vˆλ is constrained so

that its addition to λUˆ leads to the density for the real system, i.e. the wavefunction

λ

ˆ

= ρλ=1 (r) = ρ(r). Equivalently,

ψ λ (X) is such that the expectation ψ λ |ρ(r)|ψ

the ground state density is independent of λ. For each value of λ, the energy E λ is

Eλ = T λ +

λ

ρ(r)vλ (r)dr + E ee

,

(5.30)

λ

= ψ λ |λUˆ |ψ λ , the electron–

where T λ = ψ λ |Tˆ |ψ λ is the kinetic energy, and E ee

interaction energy. The equivalent constrained search definition of ψ λ (X) is that it

is an antisymmetric wavefunction which yields the density ρ(r) and minimizes the

expectation ψ λ |Tˆ + λUˆ |ψ λ .

The λ = 0 case corresponds to the S system of noninteracting fermions defined

by the differential equation (4.76). The potential energy (4.75) of these fermions is

vs (r) = vλ=1 (r) + vee (r). Since the density ρ(r) is independent of λ, we may also

write

λ

(r).

(5.31)

vs (r) = vλ (r) + vee

The ground state energy E λ may then be expressed as

E λ = Ts +

λ

ρ(r)vλ (r)dr + E ee

+ Tcλ

(5.32)

192

5 Physical Interpretation of Kohn–Sham Density Functional Theory …

or as

Eλ =

i

λ

λ

ρ(r)vee

(r)dr + E ee

+ Tcλ ,

(5.33)

i

where Tcλ = T λ − Ts is the Correlation–Kinetic energy for coupling strength λ.

5.2.1 Q–DFT Within Adiabatic Coupling Constant

Framework

The ‘Quantal Newtonian’ first law and integral virial theorem derived for the fully

interacting case are equally valid for the adiabatically coupled system. Thus, the

corresponding Q–DFT equations are the same as described in Chap. 3 but with the

appropriate λ dependence. (The S system components of these equations remain

unchanged.) The Q–DFT equations within the adiabatic coupling constant framework

are summarized below.

The pair–correlation density g λ (rr ) quantal source is

ˆ

)|ψ λ /ρ(r),

g λ (rr ) = ψ λ | P(rr

= ρ(r ) +

ρλxc (rr

(5.34)

),

= ρ(r ) + ρx (rr ) +

(5.35)

ρλc (rr

),

(5.36)

where the Fermi–Coulomb ρλxc (rr ), Fermi ρx (rr ), and Coulomb ρλc (rr ) holes satisfy

the charge conservation sum rules

ρλxc (rr )dr = −1;

ρλc (rr )dr = 0.

ρx (rr )dr = −1;

(5.37)

The Fermi hole ρx (rr ) = −|γs (rr )|2 /2ρ(r), where γs (rr ) is the S system Dirac

density matrix. The spinless single–particle density matrix source of the adiabatically

ˆ

)|ψ λ .

coupled system is γ λ (rr ) = ψ λ |γ(rr

λ

The electron–interaction field E ee (r) is then

g λ (rr )(r − r )

,

|r − r |3

= λE H (r) + λE λxc (r),

E λee (r) = λ

= λE H (r) + λE x (r) +

λE λc (r),

(5.38)

(5.39)

(5.40)

5.2 Adiabatic Coupling Constant Scheme

193

where

E H (r) =

E x (r) =

ρ(r )(r − r )

dr ; E λxc (r) =

|r − r |3

ρx (rr )(r − r )

dr ; E λc (r) =

|r − r |3

ρλxc (rr )(r − r )

dr ;

|r − r |3

ρλc (rr )(r − r )

dr .

|r − r |3

(5.41)

(5.42)

The Correlation–Kinetic field Z λtc (r) is

Z λtc (r) =

1

[z s (r; [γs ]) − z λ (r; [γ λ ])],

ρ(r)

(5.43)

λ

λ

where the component z αλ (r) of the field z λ (r) is z αλ = 2 β ∂tαβ

(r)/∂rβ , and tαβ

(r)

1

λ

2

2

is the kinetic energy density tensor tαβ (r) = 4 (∂ /∂rα ∂rβ + ∂ /∂rβ ∂rα )

γ λ (r r )|r =r =r . The field z s (r) is similarly derived from the idempotent Dirac density matrix γs (rr ) via the S-system tensor ts,αβ (r).

For the system of electrons defined by the Schrödinger equation (5.29), the

λ

(r) of the S system is the work done to

electron–interaction potential energy vee

move the model fermion in the conservative field F eff,λ (r):

λ

(r) = −

vee

where

r

F eff,λ (r ) · d ,

F eff,λ (r) = E λee (r) + Z λtc (r),

(5.44)

(5.45)

and ∇ × F eff,λ (r) = 0. For systems of symmetry such that the fields E λee (r) and

λ

(r) may be written as

Z λtc (r) are separately conservative, the potential energy vee

λ

vee

(r) = Weeλ (r) + Wtλc (r)

= λWH (r) +

(5.46)

λ

λWxc

(r)

= λWH (r) + λWx (r)

+ Wtλc (r)

+ λWcλ (r)

(5.47)

+ Wtλc (r),

(5.48)

λ

where Weeλ (r), WH (r), Wxc

(r), Wx (r), Wcλ (r), Wtλc (r) are, respectively, the work done

λ

in the fields E ee (r), E H (r), E λxc (r), E x (r), E λc (r), and Z λtc (r). The work done WH (r)

in the Hartree field E H (r) may also be expressed as WH (r) = dr ρ(r )/|r − r |.

λ

λ

, Hartree E Hλ , Pauli–Coulomb E xc

, Pauli E xλ , Coulomb

The electron–interaction E ee

λ

λ

E c , and Correlation–Kinetic Tc energies are expressed in integral virial form in terms

of the respective fields as:

λ

=

E ee

drρ(r)r · E λee (r);

(5.49)

194

5 Physical Interpretation of Kohn–Sham Density Functional Theory …

E Hλ = λE H ,

(5.50)

with

EH =

drρ(r)r · E H (r);

drρ(r)r · E λxc (r);

λ

E xc

E xλ = λE x ,

(5.51)

(5.52)

(5.53)

with

Ex =

drρ(r)r · E x (r);

(5.54)

E cλ = λ

drρ(r)r · E λc (r);

(5.55)

1

2

drρ(r)r · Z λtc (r).

(5.56)

and

Tcλ =

Note that these energy expressions are valid irrespective of whether or not the individual fields are conservative. Observe that the Hartree E Hλ and Pauli E xλ energies

scale linearly with λ.

5.2.2 KS–DFT Within Adiabatic Coupling Constant

Framework

KS–DFT employs the fact that the wavefunction ψ λ (X) of the adiabatically coupled

system is a functional of the density ρ(r). Hence, the ground state energy E λ as

obtained from the model S system is expressed as

E λ [ρ] = Ts +

KS,λ

drρ(r)vλ (r) + E ee

[ρ],

(5.57)

KS,λ

[ρ] is the KS electron–interaction energy functional. The energy funcwhere E ee

KS,λ

[ρ] is further divided into the Hartree E Hλ [ρ] and KS ‘exchange–

tional E ee

KS,λ

[ρ] components, the latter functional being further subdivided into

correlation’ E xc

an ‘exchange’ E xKS,λ [ρ] and ‘correlation’ E cKS,λ [ρ] component. Thus,

KS,λ

KS,λ

[ρ] = E Hλ [ρ] + E xc

[ρ],

E ee

=

E Hλ [ρ]

+

E xKS,λ [ρ]

+

(5.58)

E cKS,λ [ρ].

(5.59)

5.2 Adiabatic Coupling Constant Scheme

195

Here E Hλ [ρ], the Coulomb self–energy, is

E Hλ [ρ] = λE H [ρ],

with

E H [ρ] =

(5.60)

ρ(r)ρ(r )

drdr ,

|r − r |

1

2

(5.61)

and [18]

E xKS,λ [ρ] = λE xKS [ρ],

with

E xKS [ρ] =

1

2

ρ(r)ρx (rr )

drdr .

|r − r |

(5.62)

(5.63)

The Hartree functional E H [ρ] is obviously equivalent to the Q–DFT Hartree energy

E H of (5.51). The KS ‘exchange’ energy functional E xKS [ρ] expression is also equivalent to the Q–DFT Pauli energy E x of (5.54) since the source for these energies—the

Fermi hole or Dirac density matrix—is the same provided the orbitals are the same.

As a consequence, the scaling of these functionals with λ is also linear.

λ

(r) of the S system is

In KS–DFT, the electron–interaction potential energy vee

KS,λ

defined as the functional derivative of E ee (r), so that

KS,λ

[ρ]

δ E ee

δρ(r)

λ

= vHλ (r) + vxc

(r)

λ

(r) =

vee

=

vHλ (r)

+

where

vxλ (r)

+

(5.64)

(5.65)

vcλ (r),

(5.66)

δ E Hλ [ρ]

,

δρ(r)

(5.67)

vxλ (r) =

δ E xKS,λ [ρ]

.

δρ(r)

(5.68)

vcλ (r) =

δ E cKS,λ [ρ]

.

δρ(r)

(5.69)

vHλ (r) =

Here vxλ (r) and vcλ (r) are the KS ‘exchange’ and ‘correlation’ potential energies,

respectively. From the scaling relationships for E Hλ [ρ] and E xKS,λ [ρ], we see that the

corresponding functional derivatives also scale linearly [18]:

vHλ (r) = λvH (r),

(5.70)

196

5 Physical Interpretation of Kohn–Sham Density Functional Theory …

with

vH (r) =

and

δ E H [ρ]

=

δρ(r)

ρ(r )

dr ,

|r − r |

vxλ (r) = λvx (r),

with

vx (r) =

δ E xKS [ρ]

.

δρ(r)

(5.71)

(5.72)

(5.73)

As noted previously, the functional derivative vH (r) = WH (r).

5.2.3 Q–DFT and KS–DFT in Terms of the Adiabatic

Coupling Constant Perturbation Expansion

The relationship between the KS ‘exchange’ E xKS [ρ] and ‘correlation’ E cKS [ρ] energy

functionals, their functional derivatives vx (r) and vc (r), and the fields of Q–DFT

is achieved [15, 16] by expressing these fields in terms of the coupling constant

perturbation expansion. Thus, the wavefunction ψ λ (X) is expanded as

ψ λ (X) =

{φi }(X) + λψ1 (X) + λ2 ψ2 (X) + . . .

(5.74)

where {φi } is the Slater determinant of the S system. The resulting pair–correlation

density g λ (rr ) and single–particle density matrix γ λ (rr ) are

and

g λ (rr ) = gs (rr ) + λg1c (rr ) + λ2 g2c (rr ) + . . . ,

(5.75)

γ λ (rr) = γs (rr ) + λγ1c (rr ) + λ2 γ2c (rr ) + . . . ,

(5.76)

gs (rr ) = ρ(r ) + ρx (rr ),

(5.77)

where

ˆ

ˆ 1 , etc. The fields E λee (r) of (5.38) and Z λt (r) of

+ ψ| O|ψ

and O1c (rr ) = ψ1 | O|ψ

c

(5.43), which arise from these sources, are then

E λee (r) = λE H (r) + λE x (r) + λ2 E c,1 (r) + λ3 E c,2 (r) + . . . ,

and

Z λtc (r) = −λZ tc ,1 (r) − λ2 Z tc ,2 (r) − λ3 Z tc ,3 (r) − . . . ,

(5.78)

(5.79)

5.2 Adiabatic Coupling Constant Scheme

197

where E c,1 (r) = dr g1c (rr )(r − r )/|r − r |3 and Z tc ,1 (r) = z(r; [γ1c ])/ρ(r), etc.

The expansions of these fields can then be employed in the expressions for the

electron–interaction and correlation–kinetic energy and potential energy.

For systems with symmetry such that the individual fields are conservative, the

work done Weeλ (r) and Wtλc (r) in the fields E λee (r) and Z tλc (r), respectively, may then

be expressed as

Weeλ (r) = λWH (r) + λWx (r) + λ2 Wc,1 (r) + λ3 Wc,2 (r) + . . . ,

and

Wtλc (r) = −λWtc ,1 (r) − λ2 Wtc ,2 − . . . ,

(5.80)

(5.81)

where Wc,1 (r), Wtc ,1 (r), etc. are the work done in the fields E c,1 (r), Z tc ,1 (r), respectively, etc.

The scaling relationship for the KS–DFT functionals E Hλ [ρ] and E xKS,λ [ρ] are

given in the previous section. It has further been shown [17] that the KS ‘correlation’

energy functional E cKS,λ [ρ] commences in second order:

KS

KS

[ρ] + λ3 E c,3

[ρ] + . . . ,

E cKS,λ [ρ] = λ2 E c,2

(5.82)

so that the KS correlation potential too commences in second order:

vcλ (r) = λ2 vc,2 (r) + λ3 vc,3 (r) + . . . ,

(5.83)

KS

KS

[ρ]/δρ(r), etc., and E c,2

[ρ] is the O(λ2 ) KS correlation energy.

where vc,2 (r) = δ E c,2

From (5.44) we have

λ

(r) = −F eff,λ (r),

(5.84)

∇vee

so that on substituting for the field F eff,λ (r) from (5.45) and the KS definition for

the potential vee (r) from (5.66), we have

∇[vHλ (r) + vxλ (r) + vcλ (r)] = −[E λee (r) + Z λtc (r)].

(5.85)

On substitution of the expansions for the various terms as given in the previous

section, and on equating terms of equal order, we obtain the components of the KS

potential in terms of the fields as:

∇v H (r) = −E H (r),

(5.86)

∇vx (r) = −[E x (r) − Z tc ,1 (r)],

∇vc,2 (r) = −[E c,1 (r) − Z tc ,2 (r)],

(5.87)

(5.88)

∇vc,3 (r) = −[E c,2 (r) − Z tc ,3 (r)], etc.

(5.89)

198

5 Physical Interpretation of Kohn–Sham Density Functional Theory …

We are now in the position to provide a rigorous interpretation of the KS ‘exchange’

and ‘correlation’ energy functionals and their functional derivatives in terms of the

electron correlations that contribute to them. However, prior to explaining the interpretation, note that (5.86) is equivalent to (5.17). The physical reason for this equivalence between the functional derivative v H (r) and the field E H (r) is that the latter

is due to the density ρ(r) which is a static charge distribution.

5.3 Interpretation of the Kohn–Sham ‘Exchange’ Energy

Functional ExKS [ρ] and Its Derivative vx (r)

The physical interpretation of the KS ‘exchange’ potential energy vx (r) follows from

(5.87). It is the work done to move the model fermion in a conservative field R(r):

vx (r) =

δ E xKS [ρ]

=−

δρ(r)

r

R(r ) · d ,

(5.90)

where R(r) = E x (r) − Z tc ,1 (r). Since ∇ × R(r) = 0, this work done is path independent. The field R(r), and hence the potential energy vx (r), is therefore representative both of Pauli correlations via the component field E x (r), as well as those due

to part of the Correlation–Kinetic effects through the field Z tc ,1 (r).

For systems with symmetry such that the fields E x (r) and Z tc ,1 (r) are separately

conservative, we may write

vx (r) = Wx (r) + Wtc ,1 (r),

(5.91)

where Wx (r) is the work done in the field E x (r) due to the Fermi hole charge, and

Wtc ,1 (r) the work done in the field Z tc ,1 (r).

The KS ‘exchange’ energy functional E xKS [ρ] is related to its functional derivative

vx (r) by the virial theorem of (4.99). Substituting (5.87) into this equation leads to

E xKS [ρ] −

ρ(r)r · [E x (r) − Z tc ,1 (r)]dr = 0.

(5.92)

Now as noted in Sect. 5.2.2, E xKS [ρ] is equivalent to the Q–DFT Pauli energy E x

provided the same orbitals are employed in their determination. Thus, using the

relationship [19] between E x and E x (r) of (5.54) in the above equation, it follows

that

(5.93)

ρ(r)r · Z tc ,1 (r)dr = 0.

Therefore, although the Correlation–Kinetic field Z tc ,1 (r) contributes explicitly to

the potential energy vx (r), it does not contribute directly to the KS ‘exchange’ energy

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1 Interpretation of the Kohn--Sham Electron--Interaction Energy Functional EeeKS [ρ] and Its Derivative vee(r)

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