51615 Many-body approaches to studies of electronic systems: Hartree-Fock theory and Density Functional Theory
where the single-particle functionsψiare the solutions of the Hartree equations and the index
Hrefers to the density obtained by solving the Hartree equations. Check that the density is
normalised to ∫
d^3 rρH(r) =N.
Compare this density with the corresponding density ρHF(r)you get by solving the full
Hartree-Fock equations. Compare both the Hartree and Hartree-Fock densities with those
resulting from your best VMC calculations. Discuss your results.
- A popular approximation to the exchange potential in the density functional is to approximate
the contribution to this term by the corresponding result from the infinite electron gas model.
The exchange term reads then
Vx(r) =−
(
3
π
) 1 / 3
ρH(r).
Use the Hartree results to compute the total ground state energy of beryllium with the above
approximation to the exchange potential. Compare the resulting energy with the resulting
Hartree-Fock energy.
15.3.We consider a system of electrons confined in a pure two-dimensional isotropic har-
monic oscillator potential, with an idealized total Hamiltonian given by
Ĥ=
N
∑
i= 1
(
−^1
2
∇^2 i+^1
2
ω^2 r^2 i
)
+∑
i<j
1
ri j
,
where natural units (h ̄=c=e=me= 1 ) are used and all energies are in so-called atomic units
a.u. We will study systems of many electronsNas functions of the oscillator frequencyω
using the above Hamiltonian. The Hamiltonian includes a standard harmonic oscillator part
Ĥ 0 =
N
∑
i= 1
(
−^1
2
∇^2 i+^1
2
ω^2 r^2 i
)
,
and the repulsive interaction between two electrons given by
Ĥ 1 =∑
i<j
1
ri j
,
with the distance between electrons given byri j=
√
r 1 −r 2. We define the modulus of the po-
sitions of the electrons (for a given electroni) asri=
√
r^2 ix+r^2 iy. We limit ourselves to quantum
dots withN= 2 andN= 6 electrons only.
- The first step is to develop a code that solves the Kohn-Shamequations forN= 2 andN= 6
quantum dot systems with frequenciesω= 0. 01 ,ω= 0. 28 andω= 1. 0 ignoring the exchange
contribution. This corresponds to solving the Hartree equations. Solve the Kohn-Sham equa-
tions with this approximation for these quantum dot systemsand find the total density deter-
mined in terms of the single-particle wave functionsψias
ρH(r) =
N
∑
i= 1
|ψi(r)|^2 ,
where the single-particle functionsψiare the solutions of the approximated Kohn-Sham equa-
tions. Check that the density is normalised to
∫
d^3 rρH(r) =N.
