Computational chemistry is the branch of theoretical chemistry whose major goals are to create efficient computer programs that calculate theproperties of molecules (such as total energy, dipole moment, vibrational frequencies) and to apply these programs to concretechemical objects. It is also sometimes used to cover the areas of overlap between computer science and chemistry.
In theoretical chemistry chemists and physicists together develop algorithms and computer programs to allow precisepredictions of atomic and molecular properties and/or reaction paths for chemical reactions. Computational chemists in contrastmostly "simply" use existing computer programs and methodologies and apply these to specific chemical questions.
There are two different approaches in this:
There are several major areas within this topic:
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 includeempirical or semi-empirical parameters in their equations are called ab initio methods. The most popular classes of ab initio methods are: Hartree-Fock , Moller-Plesset perturbation theory , configurationinteraction , coupled cluster , reduced densitymatrices and density functional theory . Eachclass contains several methods that use different variants of the corresponding class, typically geared either to calculating aspecific molecular property, or, to application to a special set of molecules. The abundance of these approaches shows that thereis no single method suitable for all purposes.
It is, in principle, possible to use one exact method (for example, fullconfiguration interaction ) and apply it to all the molecules, but, although such methods are well-known and available in manyprograms, the computational cost of their use grows factorially (even faster than exponentially ) in the number of electrons that the moleculehas. Therefore a great number of approximate methods strive to achieve the best trade-off between accuracy and computationalcost. Presently computational chemistry can routinely and very accurately calculate the properties of the molecules that containno more than, say, 10 electrons. The treatment of molecules that contain a few dozen electrons is practically feasible only bymore approximate methods, such as DFT .
There is some dispute within the field on whenether the latter methods are sufficient to accurately describe complex chemicalreactions, such as those in biochemistry.
A number of software packages that are self-sufficient and include many quantum-chemical methods are available. Among the mostwidely used are GAUSSIAN , GAMESS , Q-Chem , NWChem , ACES , MOLPRO , DALTON , Spartan and PSI
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