"Most theoreticians peel off layers of a system until they reach the size they can still calculate in a reasonable amount of time," says Morokuma. Usually they can encompass only the reaction center, which is only the first layer of a reaction. This gives them the essence of the reaction but they miss subtle effects of environmental and structural changes. It is, says Morokuma, like trying to understand a hurricane by studying only the eye of the storm.
ONIOM captures more layers because it uses calculations in each layer that are less expensive but still appropriate to the degree of influence the region has on the reaction. For instance, the second layer captures electronic effects -- that is, the exchange of electrons among neighboring atoms. Because this movement affects the speed of the reaction, it demands fairly reliable, moderately expensive calculations. The third layer is far from the reaction center. The energies of these distant molecules can be obtained through cheap molecular mechanical calculations.
The ingenious part of ONIOM is how it combines the results of different calculations so that the results can be extrapolated to an entire system. First a high-level calculation is performed on the reaction center and added to a low-level calculation on the entire system. Then a low-level calculation performed on the reaction center is subtracted from the total.
When Morokuma compared the results from his ONIOM calculations with arduous, detailed calculations run on many compounds, the results were nearly identical.
The results didn't surprise Morokuma.
"It was so simple," says Morokuma, "and when we told people about it they said, 'Oh, that's obvious.' Of course it is obvious. That's why it is beautiful."
