Example of principal component analysis

A chemist is normally visualized as someone in a white laboratory coat performing an experiment. However, some chemists never acttially go into a wet chemical laboratory but utilize computers to perform experiments. 

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. 

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. 

A quantum-mechaiucal calculation commences with the Schrodinger wave equation  

The equation is used to describe the behaviour of an atom or molecule in terms of its wave-like (or quantum) nature. By trying to solve the equation the energy levels of the system are calculated. However, the complex nature of multielectron/nuclei systems is simplified using the Born-Oppenheimer approximation. Unfortunately it is not possible to obtain an exact solution of the Schrddinger wave equation except for the simplest case, i.e. hydrogen. Theoretical chemists have therefore established approaches to find approximate solutions to the wave equation. One such approach uses the Hartree-Fock self-consistent field method, although other approaches are possible. Two important classes of calculation are based on ab initio or semi-empirical methods. Ah initio literally means from the beginning . The term is used in computational chemistry to describe computations which are not based upon any experimental data, but based purely on theoretical principles. This is not to say that this approach has no scientific basis - indeed the approach uses mathematical approximations to simplify, for example, a differential equation. In contrast, semi-empirical methods utilize some experimental data to simplify the calculations. As a consequence semi-empirical methods are more rapid than ab initio. 

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