details of what happens at the bottom of the active site in the enzyme, we will need to use quantum mechanics.

The question is, If you start using quantum mechanics there, where do you switch over as you move out in space to a point where you can go over to a lower-level treatment, a semi-empirical or empirical method? There has been a lot of effort in that recently. Some efforts have been more successful than others. I think this is something we will work out over the next few years, and I think this is the real future. Ideally there should be no need to treat a system like that entirely quantum mechanically. If we could do it cheaply enough, well, then we could do it that way. But in practice I do not think that should be necessary.

But I think we do not completely understand yet how to splice these different levels of treatment together. How do we go, for example, from a very accurate quantum chemistry calculation for an area where we are really interested in the details to a lower-level quantum chemistry calculation, say a density functional calculation or something in a larger region, and then eventually to some completely empirical type of molecular mechanics approach in the next layer out?

Thom Dunning: Let me make one addition to that statement. One of the surprises that came out of the ASCI program was that the increased computing power makes it possible—even though, as Peter pointed out, the techniques that we use right now do not scale very well with the size of the system—to actually compute all of the thermodynamics for all of the species and the reactions involved in combusting both gasoline and diesel fuel. And, in gasoline there are some 1,000 species that are thought to be involved, and some 2,000 reactions that the modelers say they need to have kinetic data for.

For diesel fuel you are talking about 2,000 species and 4,000 reactions. And some of those species have not even been seen or characterized in the laboratory yet, so even with the techniques as they are right now, one can, without increasing computing power, actually make substantial contributions to very practical problems that the country faces. But, I would agree entirely with Peter in that we need techniques that scale better with the size of the system.



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