Not all models are physical or pictorial objects. For example, the SN2 mechanism is a simple model for a particular class of reactions that successfully explains a lot of chemistry. What all of these things have in common is that they use a set of pre-defined objects and rules to approximate real chemical entities and process.
In a similar way, computational chemistry simulates chemical structures and reactions numerically, based in full or in part on the fundamental laws of physics. It allows chemists to study chemical phenomena by running calculations on computers rather than by examining reactions and compounds experimentally. Some methods can be used to model not only stable molecules, but also short -lived, unstable intermediates and transition states. In this way, they can provide information about molecules and reactions which is impossible to obtain through observation.
There are two broad areas within computational chemistry devoted to the structure of molecules and their reactivity : molecular mechanics and electronic structure theory. They perform the same basic types of calculations :
Molecular mechanics calculations do not explicitly treat the electrons in a molecular system. Instead, they perform computations based upon the interactions among nuclei. Electronic effects are implicitly included in force fields via its parametrization.
This approximation makes molecular mechanics computations quite inexpensive in computation time, and allows them to be used for very large systems containing thousands of atoms. However, it also carries several limitations :
|H = E||(1)|
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