Practical Considerations and Nomenclature

TABLE 1. Computer times and geometries of H3SiF calculated using various theoretical methods 

0 FO = geometry fully optimized assuming C3v symmetry SP = single-point calculation at a specified geometry. b In minutes. 

Finally, it may be appropriate to add a note of caution for the nonexpert. Even when the most sophisticated techniques are used, the assumptions and simplifications that have to be made for a molecule of any complextiy are still quite severe. Thus, the absolute errors in total energies are huge, by far larger than any chemical phenomena that we may wish to describe. However, the vast experience that has accumulated in the last two decades has shown that in spite of the many uncertainties involved in the results of quantum-mechanical computations, a careful use of the methods can produce extremely useful results. In fact, in some cases discussed below the calculations will serve to point out erroneous experimental interpretations. This review will emphasize the usefulness, strength and credibility of the computational methods. However, as in carrying out experiments, also when performing calculations one must keep in mind the assumptions and pitfalls involved in the specific method used. 

An additional important point that one should bear in mind is that the calculations describe explicitly only the isolated molecule in the gas phase (in some cases, simulation of solvation is attempted but such calculations are still relatively rare). It is now well established that in many cases solvation affects drastically a wide variety of molecular and chemical properties and could even reverse experimental trends, relative stabilities, etc61. This point should be kept is mind whenever comparing calculations with experimental data, particularly when charged or highly polar molecules are involved. For energies we use hartrees or kcalmol-1 throughout. 1 hartree = 2625.54 kJmol-1 = 627.52 kcal mol i. 

0 Commission on Atomic Weights and Isotopic Abundances, Pure Appl. Chem,., 55, 1101 (1983) 56, 653 (1984). 

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