Computer Determination of Molecular Properties for Diatomics
To understand and characterise matter, we determine as completely as possible the different properties of atoms and molecules to gain insight into their behavior; how they interact and react. The determination of molecular properties can be accomplished experimentally, by performing the appropriate measurement in the laboratory, or theoretically by using the laws of quantum mechanics to compute the desired property. In principle, the experiment will obviously yield the correct result, while the theoretical determination rests on our belief that quantum mechanics correctly describes atomic and molecular structure. In practice, both procedures are frought with difficulties, which are, however, quite different for the two methods. While many properties of a wide variety of molecules can be determined with ease by experiment, some are more readily and reliably obtained by computer. To be sure, the experimental approach is way ahead; experimentalists have determined molecular properties for more than a century, contributing greatly to our understanding of molecular structure and to the development of the laws of quantum mechanics. Nevertheless, there are cases, where the experimental approach breaks down or is severely hampered by difficulties, especially when the molecules in question are unstable or difficult to make. These present no limitations to the theoretical approach, since any molecule can be made and kept stable inside a computer. Here the problems and difficulties are of a different nature. The equations to be solved, and the quantities to be computed for a reliable theoretical determination of molecular properties are so complex that availability of high speed electronic computers was required before it became feasible to travel the theoretical avenue. Computers are young and the availability of machines with large enough capacity and high enough speed still more recent and limited. It is therefore not surprising that theoretical determination of molecular properties lags well behind experimental measurement.
KeywordsQuantum Number Molecular Property Internuclear Distance Adiabatic Approximation Spectroscopic Constant
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