Quantum Mechanical Calculations on Small Molecules

This Report deals with recent theoretical work on small molecules, and covers the literature to 1 July 1974. Discussion is restricted to molecules containing up to four atoms, and work on molecules containing approximately 5—12 atoms will be reviewed in Vol. 3. 

This Report does not aim to be in any sense comprehensive, especially since the literature on quantum chemistry is expanding so rapidly, and discussion is restricted to those results in the field of calculations on small molecules which are judged by this Reporter to be of particular importance. Both non-empirical (ab initio) and semi-empirical calculations will be dealt with, but since most work on small molecules in recent years has used ab initio methods, this type of calculation will be emphasized. 

Most of the papers cited refer to work published during the period 1972—74, but there are also references to work of particular importance prior to 1972 if necessary. 

Developments in the theoretical methods as such will not be dealt with in any detail, the emphasis being on the results of calculations. Results are usually quoted in atomic units (Distances/bohr, energies/hartree), but there are occasional exceptions to this. 

A number of books and review articles dealing with the subject matter of this chapter should first be mentioned. The most comprehensive introduction to recent work is by Schaefer,1 whilst a more recent book on ab initio calculations, by Cook, is also recommended.2 A fairly comprehensive bibliography (up to 1974) on ab initio calculations is provided by Richards et al.3 Volume 1 of the MTP Review of Science4 deals with theoretical chemistry and contains several articles of interest, particularly an article by Wahl on diatomic molecules, and a review of the work of Pople s group. There have also been several books dealing with semi-empirical methods in 

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Quantum mechanical calculations on small molecule association suggest that there are five major contributions to the energy of intermolecular interactions in the gas phase (3, 4). The sum of these is the dissociation energy of the intramolecular complex represented in Fig. 4.1. Table 4.1 contains some examples of magnitudes of the different energy components for different interactions. This section provides a qualitative introduction to these forces. Section gives and overview of mathematical models suitable for computer calculations. 

In the past, force fields were determined from experiment. Now they may be determined either by experiment, or by quantum mechanical calculations directly, or by quantum mechanical calculations on small molecules, which are used to develop force fields for large molecules. The latter method is exceedingly powerful, now that reasonably accurate calculations on molecules of moderate size can be carried out by ab initio methods. The most reliable force fields result when one can develop the force field from both ab initio methods and from experiment, and obtain results that are acceptably accurate by both methods. 

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