Strengths and Weaknesses of Molecular Mechanics

One of the major difficulties with molecular mechanics procedures (MMh- or otherwise) is that they almost always fail. That is, you find that force constants are not available for the molecule of interest. This is both the strength and weakness of molecular mechanics it uses atom types to introduce specific chemical environments for the atoms within a molecule (to obtain accuracy in the calculations) but then requires knowledge of force constants specific to that chemical environment (as specific as stating that an atom is in a five-member ring containing one oxygen and one carbon, for example). As the number, N, of atom types rises the number of force constants needed to describe all possible occurrences of these atom type becomes very large. For torsions, for... 

List the strengths and weaknesses of ab initio calculations compared to molecular mechanics and extended Hiickel calculations. State the molecular features that can be calculated by each method. 

It is possible to use computational techniques to gain insight into the vibrational motion of molecules. There are a number of computational methods available that have varying degrees of accuracy. These methods can be powerful tools if the user is aware of their strengths and weaknesses. The user is advised to use ah initio or DFT calculations with an appropriate scale factor if at all possible. Anharmonic corrections should be considered only if very-high-accuracy results are necessary. Semiempirical and molecular mechanics methods should be tried cautiously when the molecular system prevents using the other methods mentioned. 

Chemists seeking to use computational chemistry to support experimental efforts now have three generd theoretical tools available to them force field or molecular mechanics models, ab initio molecular orbital (MO) models and semiempirical MO models (1). Each of these tools have strengths and weaknesses which must be evaluated to determine which is most appropriate for a given applications. 

As indicated before, the ab initio electronic-structure theory of solid-state materials has largely profited from density-functional theory (DFT), and the performance of DFT has turned out well even when the one of its molecular quantum-chemical competitors - Hartree-Fock theory - has been weakest, namely for metallic materials. For these, and also for covalent materials, DFT is a very reasonable choice. On the other hand, ionic compounds (with both metals and nonmetals present) are often discussed using only the ionic model, on which most of Section 1.2 was based, and the quantum-mechanical approach is not considered at all, at least in introductory textbooks. Nonetheless, let us see, as a first instructive example, how a t5q)ical ionic material can be described and understood by the ionic and the quantum-chemical (DFT and HF) approaches, and let us also analyze the strengths and weaknesses. 

Can we put this analysis on a quantitative basis Could we develop some sort of equation that relates the extent of distortion to the energy of the molecule We can, and the method is called molecular mechanics. Here we will lay out the basic tenants of molecular mechanics and provide a description of its strengths and weaknesses. The method is now quite common and easily implemented for sizable molecules on a standard personal computer. It is a powerful aid to experimentalists in all fields of organic chemistry, as well as in molecular-scale studies of biology and materials science. It should be appreciated from the start, however, that the method has significant limitations and is susceptible to misuse. 

To get an overview of the currently available software for molecular mechanics calculations with their strengths, weaknesses, and application areas... 

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