by the output of the calculation, rather than being required to be extracted, perhaps
somewhat tortuously, from experiments, (3) are usually more quickly calculated
than determined in the lab, and (4) can be uniformly secured, that is, all parameters
can be obtained from calculations at the same level, say MP2/6-311G(df,p), in
contrast to experiment, where different methods must be used to obtain different
parameters. This last point may be more of an esthetic than a utilitarian advantage.
The advantage of parameterizing with experimental quantities is that, if the
experiment is reliable, then weknowthat the values of the parameters; we need
not reflect on the reliability of the calculation. Of course, we might wish to ponder
the accuracy of the experiment.
Reference
- Lewars E (2008) Modeling marvels. Computational anticipation of novel molecules. Springer,
The Netherlands
Chapter 3, Harder Questions, Answers
Q4
Would you dispute the suggestion that no matter how accurate a set of MM results
might be, they cannot provide insight into the factors affecting a chemical problem,
because the “ball and springs” model is unphysical?
First, the ball and springs model used in molecular mechanics is not completely
nonphysical: to a fair approximation, molecules really do vibrate and bonds do
stretch and bend, as expected from a macroscopic ball and springs model. It is when
we want to examine inescapably electronic properties, like, say, UV spectra or the
donation of electrons from one species to another to make a bond, that the MM
model is completely inadequate.
Since MM gives geometries that vary from fairly to highly accurate for molecules
that are not too outre ́, where steric factors are relevant itcanprovide chemical insight.
Chapter 3, Harder Questions, Answers
Q5
Would you agree that hydrogen bonds (e.g. the attraction between two water
molecules) might be modelled in MM as weak covalent bonds, as strong van der
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