Quantum Chemistry and Small Molecule Dynamics

Abstract

Theoretical calculations have now reached the point where they must be, under certain circumstances, considered a reasonable alternative to experiment and certainly as a means of obtaining information which is complimentary to experiment, particularly for systems that involve, for instance, very short-lived transients, very extreme environments, very high temperatures, etc. In most cases the actual systems involve areas where experimental information is difficult to obtain, or where there is significant uncertainty in the experimental information available. Oddly enough, even small, such fairly well-known systems as the atmospheric molecules? fall into this class. One would think these molecules would already be very well characterized but they are not; their excited states are virtually undetermined and often the experimental data is certainly no more reliable than the calculations. Most of theoretical chemistry hinges on our ability to calculate both the energy of interaction as atoms and molecules come together and the difference between energies as you excite or ionize the system. It has really been the improvement over the past five years in our ability to accurately evaluate these potential curves and excitation energies that has brought theoretical chemistry to a point where it can, for small and selected systems, provide information at might be called chemical accuracy. Most of us now agree that chemical accuracy in broad areas of chemistry lies in the range of a few kcal/mole. Certainly in parts of spectroscopy one goes much higher in accuracy, but for many systems the spectra are unknown and even to have partial answers within that accuracy is useful. In figure 1a and 1b* we see results obtained in the mid 1960 using the molecular orbital model. This model gives us a very consistent conceptual framework for thinking about molecules, atoms and molecular processes; a way of talking about the differences between molecules and geometries, particularly how atomic systems evolve into molecules (figure 1c). This model and extensions based on it form the basis for what might be called modern computational chemistry: the basis for implementing on computers a large mathematical model which has general applicability to molecules of every sort.