Computational chemistry

Computational chemistry is the branch of theoretical chemistry whose major goals are to create efficient computer programs that calculate the properties of molecules (such as total energy, dipole moment, vibrational frequencies) and to apply these programs to concrete chemical objects.

The programs used in computational chemistry are based on many different quantum-chemical methods that solve the molecular Schrödinger equation. The methods that do not include empirical or semi-empirical parameters in their equations are called ab-initio methods and are currently of the greatest use in computational chemistry. The most popular classes of ab initio methods are: Hartree-Fock, Moller-Plesset Perturbation Theory, Configuration Interaction, Coupled Cluster, Reduced Density Matrices and Density Functional Theory. Each class contains several methods that use different variants of the of the corresponding class, typically geared either to calculating a specific molecular property, or, to application to a special set of molecules. The abundance of these approaches shows that there is no single method suitable for all purposes.

It is, in principle, possible to use one exact method (for example, Full Configuration Interaction) and apply it to all the molecules, but, although such methods are well-known and available in many programs, the computational cost of their use grows factorially (even faster than exponentially) in the number of electrons that the molecule has. Therefore a great number of approximate methods strive to achieve the best trade-off between accuracy and computational cost. Presently computational chemistry can routinely and very accurately calculate the properties of the molecules that contain no more than, say, 10 electrons. The treatment of molecules that contaion a few dozen electrons is practically feasible only by very approximate methods. It is not seen how in the near future it would be possible to accurately calculate properties of even slightly larger systems. The opinion that computational chemistry would be ultimately able to predict mechanisms of such complex processes as biochemical reactions is now looked upon as unjustifiably optimistic. For the time being, one attempts to address the latter using molecular dynamics simulations.

A number of software packages that are self-sufficient and include many quantum-chemical methods are available. Among the most widely used are GAUSSIAN, GAMESS, Q-Chem, ACES, MOLPRO, Spartan and PSI.