Introduction to special issue

Theoretical Possibilities

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Science  08 Aug 2008:
Vol. 321, Issue 5890, pp. 783
DOI: 10.1126/science.321.5890.783

Not so long ago, Experiments in the synthetic chemistry lab tended to proceed without detailed calculations and predictions. Today, most organic chemists rely on the results of density functional theory (DFT) calculations to help decide between alternative synthetic routes and to provide guidance on structure assignments. However, such readily solved problems are not where theoretical chemists focus their attention for their own current day-to-day work—rather, it is on the problems that still defy solution. This special section highlights some areas where theorists feel that progress can be made in the near future on problems that will benefit their experimental colleagues in chemistry and other disciplines.

Stone (p. 787) describes the ubiquitous first step in tackling theoretical chemistry problems: constructing a potential energy surface, which describes the energy of the molecule or assembly of molecules as a function of the atomic positions. Clary (p. 789) goes on to detail the status of predicting reactions between isolated small molecules in the gas phase—a context in which the highest level of quantum-mechanical detail is currently attainable. Reactions involving three atoms have been modeled successfully in this way, but even four- or six-atom reactions remain challenging. Although DFT has been the workhorse of most computational studies, it is not reliable in certain classes of problems; for example, it often underestimates transition-state barriers in reactions or band gaps of materials. Cohen et al. (p. 792) trace the problems of many DFT approximations to delocalization and static correlation errors through a framework that makes use of fractional charges and fractional spins.

Practical chemistry often involves molecular interactions in multiple phases beyond gaseous collisions. Kroes (p. 794) discusses progress in modeling molecule/surface interactions, which play a central role in heterogeneous catalysis. Much effort has focused on unraveling the contexts in which electronic and nuclear motion become coupled in time. Klein and Shinoda (p. 798) describe simulations of complex molecular systems, in which a vast number of simultaneous interactions are encompassed through molecular dynamics trajectories. Use of so-called coarse-grain models allows the behavior of micrometer-scale systems of polymers or biopolymers to be modeled over the course of microseconds, a time scale that is highly relevant for comparison to experiments.

Theoretical chemistry is now commonly used to address complex problems in biochemistry and materials science. In a News story, Service (p. 784) describes recent successes in simulating protein folding, a problem long hindered by the computational intractability of the immense number of accessible configurations. Carter (p. 800) moves beyond molecular systems to focus on simulations of extended materials. Recent progress in ab initio and DFT methods has facilitated purely theoretical explorations of features ranging from mechanical properties to corrosion behavior. Electronic excitation remains a challenging frontier.

A recurring theme in all of these articles is the complementary role of theory and experiment in exploring chemical questions. Each approach nourishes the other, presenting fresh challenges.

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