Computational chemistry





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

The programmes used in computational chemistry are based on many different
quantum-chemical methods that solve the molecular Schršdinger 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
calcualting 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 programmes, the
computational cost of their use grows factorially (not even 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.