Chemistry Reference
In-Depth Information
investigations of hydrogen bonding (reminiscent of those outlined above) were
conducted approximately four decades ago.
85,110,111
Accuracy has been and
continues to be one of the major challenges facing theoreticians (as well as
experimentalists) working with weakly bound clusters. Consider, for example,
covalent versus noncovalent interactions. An error of a few kilojoules per mole
(
chemical accuracy
) per covalent bond may be acceptable because it typically
represents a relative error of just a few percent. However, for weak noncovalent
bonding an absolute error of a few kilojoules per mole could easily amount to a
relative error in excess of 100%. Fortunately, by carefully applying the arsenal
of sophisticated electronic structure techniques available today, it is possible to
reduce the major sources of error (basis sets and electron correlation) to accep-
table levels.
One of the most important lessons learned over the years is that not all
weak noncovalent interactions are created equal. A particular quantum model
chemistry that provides quantitatively reliable results for hydrogen bonding
may yield qualitatively incorrect results for something like
stacking. For
example, second-order Møller-Plesset (MP2) perturbation theory and several
popular density functional (DFT) techniques can characterize the water dimer
and trimer with a reasonable degree of accuracy. However, the former method
overestimates
p
-stacking interactions in benzene by a factor of 2, while the lat-
ter fail to yield any sort of attractive interaction between two stacked benzene
molecules. Consequently, it is imperative that ''the right answer'' is obtained
for ''the right reason'' rather than relying on (or hoping for) some sort of error
cancellation. Fortunately, well-established procedures exist by which one can
converge to ''the right answer.'' The most common of these convergent
approaches to high-accuracy computational chemistry systematically improve
(i) the correlated electronic structure techniques and (ii) the atomic orbital
(AO) basis sets. This dual extrapolation scheme is depicted in Figure 4.
p
Figure 4
Example of convergent quantum chemistry scheme that employs AO basis sets
that systematically approach the 1-particle or complete basis set (CBS) limit along with
correlated electronic structure techniques that systematically approach the
n
-particle or
full configuration interaction (FCI) limit.
