Computational chemistry is a branch of chemistry that uses computational tools (that is, specialized software) to study the characteristics and behavior of matter. These programs employ mathematical tools to calculate chemical properties such as potential energy, free energy, molecular geometry at equilibrium and transition states, dipole moment, vibrational spectra, electronic and charge distribution, etc. It is also possible to perform simulations of the dynamic behaviour of matter including, among many others, the diffusion of drugs through the cellular membrane, the dissolution of salts in water, the formation of ice crystals and the reaction between two or more reactants.
Using computational chemistry it is possible to run a large volume of calculations in a short time. Thus, it is extremely valuable for the development of new pharmaceutical drugs since it allows, for example, testing millions of molecules in order to select the ones that present a better fit within the cavity of a cellular receptor. Such task would not be feasible in the laboratory. Additionally, it is useful when studying systems that are difficult to replicate, such as fusion reactions that occur inside stars, prohibitively expensive or dangerous materials and the transition state of chemical reactions, whose lifetimes are in the nanosecond timescale. Besides all of this, studies performed using computational methods are more detailed, more ecological and cheaper than those carried out in the lab. However, computational chemistry does not attempt to replace laboratory experiments; on the contrary, it complements them, for example, aiding in the interpretation of experimental vibrational spectra.
Most of the calculations and simulations performed within computational chemistry are based on Schrödinger’s equation, a quantum physical model that describes, through mathematical algorithms, the behaviour of electrons in a chemical system (as in a molecule). Quantum chemical models are those that depict reality with more accuracy and detail, however they are quite computationally demanding, that is, a simple calculation requites a large number of mathematical operations. Consequently quantum methods, when applied to large systems, such as a hormone, require the use of computer clusters. Even when you have access to a supercomputer it is not practical to study gigantic systems, like proteins or cellular membranes, using exclusively quantum methods. In these cases faster but less accurate methods, such as molecular mechanics, which is based on classical physics, are used to describe these systems, either partially or in totality.
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http://www.shodor.org/chemviz/overview/ccbasics.html
Cramer, C., 2004, Essentials of Computational Chemistry: Theories and Models. 2nd ed: Wiley.
