Computational chemistry

A chemist is normally visualized as someone in a white laboratory coat performing an experiment. However, some chemists never actually go into a 'wet chemical' laboratory but utilize computers to perform experiments. Why do chemistry on computers?

Figure 44.1 Computer generated images of six amino acids using different representations (a) stick, (b) balls, (c) balls and cylinders and (d) sticks and dots.

• Safety: invariably all laboratory experiments carry some risk, associated with the chemicals or apparatus to be used. It is usual to perform a COSHH assessment (p. 7) prior to experimental work. Computational chemistry allows the user to carry out work on 'dangerous' chemicals with no risk!
• Cost: apart from the financial outlay on a computer and associated peripherals together with the appropriate software, no further costs are involved, unlike the e xperimental laboratory where most chemicals require disposal after use.
• Understanding: computational chemistry has the ability to provide a basis for understanding chemical principles.

This branch of chemistry, sometimes referred to as theoretical chemistry, molecular modelling or computational chemistry, used to be restricted to a few researchers with access to expensive computers. This has changed in recent years with the availability of low-cost, high-power computers, coupled with the availability of software packages that require no knowledge of computer programming, making computational chemistry accessible to undergraduate students. The new user of computational chemistry will quickly discover the ease with which it is possible to observe complex molecules on the screen using the software's computer graphics (Fig. 44.1). However, computational chemistry is much more than pretty pictures. It allows the chemist to perform theoretical experiments in three distinct areas:

1. single molecule calculations
2. molecular interactions
3. reactions of molecules.

In all cases, the basis of the calculation is the determination of the energy of the system. Two approaches are used: quantum mechanics and molecular mechanics. Molecular mechanics is best suited to large molecules, e.g. proteins, whereas quantum mechanics, while offering a more fundamental approach, is restricted to smaller molecules.

*Note: Just as in spectroscopy or chromatography, where not every spectrum or peak is resolved, so in computational chemistry do not assume every computed number is exact. However, computational chemistry can allow a qualitative or approximate insight into chemical processes provided the user understands the basis behind each approach and can interpret the results.