In seeking the solutions to the system of equations (3.17), it
is found that all quantities are represented as functionals of the
electronic charge density. The important point that makes this system
easier to solve (or more precisely, require less computation) than, for
example the Hartree-Fock equations, is that the effective potential is
local (although note Section 3.7 discussing the use of
nonlocal pseudopotentials). Therefore there is no more complexity added
in solving (3.17) than there is in the Hartree approximation. Of
course, this is only true if the exchange-correlation energy can be
described as a function of the local charge density. A method of doing
so is known as the *local density approximation*[29] (LDA). In
LDA, the exchange-correlation energy of an electronic system is
constructed by assuming that the exchange-correlation energy per
electron at a point in the electron gas, is equal to the exchange-correlation energy per electron in a
homogeneous electron gas that has the same electron density at the
point . It follows that

so that

with

where Equation (3.21) is the assumption that the exchange-correlation energy is purely local. Several parameterisations for exist, but the most commonly used is that of Perdew and Zunger[37]. This parameterisation is based on the quantum Monte Carlo calculations of Ceperley and Alder[38] on homogeneous electron gases at various densities. The parameterisation uses interpolation formulas to link these exact results for the exchange and correlation energy at many different densities.

In LDA, corrections to the exchange-correlation energy due to the inhomogeneities in the electronic charge density about are ignored. Considering this inexact nature of the approximation, it may at first seem somewhat surprising that such calculations are so successful. This can be partially attributed to the fact that LDA gives the correct sum rule to the exchange-correlation hole. That is, there is a total electronic charge of one electron excluded from the neighbourhood of the electron at . Attempts to improve on LDA, such as gradient expansions to correct for inhomogeneities do not seem to show any improvement in results obtained by the simple LDA. One of the reasons for this failure is that the sum rule is not obeyed by the exchange-correlation hole.

A summary of the contributions of electron-electron interactions in
N-electron systems is shown in Figure 3.1. It illustrates the
conditional electron probability distributions of
*N*-1 electrons around an electron with given spin situated at . In the Hartree approximation, Figure (a), all electrons are
treated as independent, therefore is structureless. Figure
(b) represents the Hartree-Fock approximation where the N-electron
wavefunction reflects the Pauli exclusion principle. Around the electron
at the exchange hole can be seen where the the density of
spins equal to that of the central electron is reduced. Electrons with
opposite spins are unaffected. In the LDA, where spin states are
degenerate, each type of electron sees the same exchange-correlation
hole (the sum rule being illustrated where the size of the hole is one
electron). Figure (d) shows electron-electron interaction for
non-degenerate spin systems (the local spin density approximation (LSD).
It can be seen that the spin degenerate LDA is simply the average of the
LSD).

**Figure 3.1:** Summary of the electron-electron interactions (excluding
coulomb effects) in (a) the
Hartree approximation, (b) the Hartree-Fock approximation, (c) the local
density approximation and (d) the local spin density approximation which
allows for different interactions for like-unlike
spins.

Thu Oct 31 19:32:00 GMT 1996