Bonding models reactivity rules based

Why then bother about much more expensive QM-based models One reason is that MM may only lead to accurate results for molecules of the same type used for the optimization and validation of the force field, i.e. extrapolation is seen to be dangerous if not impossible [9], This also extends to transition states and shortlived, unstable intermediates and therefore to chemical reactivity. Since electrons are not considered explicitly in MM, electronic effects related to structural distortions, specific stabilities and spectroscopy cannot be modeled by MM. However, in all other areas, there is no good reason for not using a well-optimized and validated MM model. Also, there are MM-based approaches to deal with most of the deficiencies listed above [9,20-28]. In the last decade, there have been a number of approaches, which have, based on simple rules [29], valence bond theory [30-33] and ligand-field theory [20-23], allowed the simplification of the force-field optimization and validation procedures and/or inclusion of electronic effects in MM models. 

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