Biomedical Engineering Reference
In-Depth Information
21
Accurate Thermodynamic Properties;
the G
n
Models
Amajor objective of quantum chemical modelling is the prediction of molecular structure
and energy. The enthalpies of formation of many molecules have been determined very
accurately by precise experimental techniques such as calorimetry and spectroscopy, but
there is a wide range of species, for example unstable transients, for which the uncertainty
is large. Molecular ionization energies, electron affinities and proton affinities have also
been well studied experimentally. Realistic calculations of thermodynamic data have to be
capable of giving 'chemical' accuracy and the target is usually set as
±
2 kcal mol
−
1
for
energy quantities.
It has been recognized for many years that bond dissociation energies are poorly predicted
by standard
ab Initio
HF-LCAO calculations. The development of MP
n
perturbation theory
marked a turning point in the treatment of molecular correlation, and made the treatment
of correlation energy at last possible for molecules of moderate size (but at a high cost
in computer resource). The MP
n
methodology is usually implemented up to MP4 level,
but the convergence of the perturbation series leaves something to be desired and for this
reason a further quadratic CI correction is usually applied.
The effect of polarization functions is also important, and even f orbitals make a
contribution to the total energies of first-row atoms.
21.1 G1 Theory
The
Gaussian-1
(
G1) method
was introduced by Pople
et al.
(1989), in order to systemat-
ically investigate the shortcomings in the levels of theory known at that time. Here is the
abstract of their paper:
