Chemistry Reference
In-Depth Information
Post-Hartree-Fock Methods
In this chapter we shall briefly introduce some methods which, mostly
starting from the uncorrelated HF approximation, attempt to reach
chemical accuracy (1 kcal mol
1
or less) in the quantum chemical calcula-
tion of the atomization energies. We shall outline first the basic principles
of configuration interaction (CI) and multiconfiguration SCF (MC-SCF)
techniques, proceeding next to some applications of the so-called many-
body perturbation methods, mostly the Møller-Plesset second-order
approximation to the correlation energy (MP2), which is the starting
point of the more efficient methods of accounting for correlation effects
directly including the interelectronic distance in the wavefunction, such as
theMP2-R12 and CC-R12 methods of the Kutzelnigg group. The chapter
ends with a short introduction to density functional theory (DFT).
8.1 CONFIGURATION INTERACTION
Given a basis of atomic or molecular spin-orbitals, we construct a linear
combination of electron configurations in the form of many-electron
Slater determinants, with coefficients determined by the Ritz method,
to give the CI wavefunction:
; ...; x
N
Þ¼
X
k
Y
ðx
1
; x
2
Y
k
ðx
1
; x
2
; ...;
x
N
Þ
C
k
ð
8
:
1
Þ
When all possible configurations arising from a given basis set are
included, we speak of the full-CI wavefunction. It should be recalled that
