Computational Chemistry

DH{¼171.5

DGreact¼#119.3

DG{¼169.2

DHreact¼#120.0

Cyclopropylidene to allene

Transition state 0 K relative E¼25.7 Product 0 K relative E¼# 288

Ea¼DH{þRT¼DH{þ2.48¼27.8

DH{¼25.3

DGreact¼#285.1

DG{¼23.9

DHreact¼#285.9

For these reactions the 0 K activation enthalpy and the room temperature
activation enthalpies and free energies are almost the same, and so are the 0 K
reaction enthalpy and the room temperature reaction enthalpies and free energies.
This is presumably so because these are unimolecular reactions, in which the
relative translational velocities of reacting molecules are not a factor.
The HF method tends to overestimate the barriers, making unstable molecules
seem stabler than they really are. Geometries are discussed further inSection 5.5.1.
Approximate versions of the MP2 method that speed up the process with little loss
of accuracy are available in some program suites: LMP2, localized MP2, and RI-
MP2, resolution of identity MP2. LMP2 starts with a Slater determinant which has
been altered so that its MOs are localized, corresponding to our ideas of bonds and
lone pairs (Section 5.2.3.1), and permits only excitations into spatially nearby
virtual orbitals [ 93 ]. RI-MP2 approximates four-center integrals (Section 5.3.2)
by three-center ones [ 94 ].

5.4.3 The Configuration Interaction Approach to Electron

Correlation – The Coupled Cluster Method

The configuration interaction (CI) treatment of electron correlation [ 83 , 95 ] is based
on the simple idea that one can improve on the HF wavefunction, and hence energy,
by adding on to the HF wavefunction terms that represent promotion of electrons
from occupied to virtual MOs. The HF term and the additional terms each represent
a particular electronic configuration, and the actual wavefunction and electronic
structure of the system can be conceptualized as the result of the interaction of these
configurations. This electron promotion, which makes it easier for electrons to
avoid one another, is as we saw (Section 5.4.2) also the physical idea behind the
Møller–Plesset method; the MP and CI methods differ in their mathematical
approaches.

5.4 Post-Hartree–Fock Calculations: Electron Correlation 269