Current development and need for extensions

The simple setting characteristic of MM facilitates its wide use and a wealth of implementations, differing in the exact set of parametric forms of the force fields employed 

MM2 [189] uses cubic anharmonic potential to represent the bond stretching, up to sixth power expansion for the valence angle bending, and harmonic field for the out-of-plane deformations. The stretch-bending cross term is included. 

The MM3 method [190] is parametrized for as much as 153 atomic types eventually covering almost all chemical elements in common use. The quartic anharmonic potential is used for the bond stretching, sixth power expansion 

The MM4 method [ 191 ] is still under development in terms of extending the number of chemical elements it is parametrized for. The initial published results are encouraging. It uses the sixth power expansion, both for the stretching and for the bending and the improper torsion representation for the out-of-plane deformations. The idea of using the bond-dipole based electrostatic term is abandoned in the MM4 whereas the whole collection of the cross terms is included. 

Several important extensions were based on the MM2 platform. Among them the MMP2 designed to incorporate the effects of the conjugate 7r-systems upon the molecular geometry and torsion barriers must be mentioned. MMX and MMa are variations of MM2. 

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