Molecular mechanics calculations, also known as force field calculations, can be of considerable use in the qualitative descriptions in these cases. In essence, we describe the potential energy surface without invoking any quantum mechanical calculations or descriptions. The Born-Oppenheimer approximation, fundamental to our molecular description, states that the Schrodinger Equation for a molecule can be separated into a part describing the motions of the electrons and a part describing the motions of the nuclei and that these two motions can be studied independently. This can be interpreted in one of two manners, one of which allows the study of the electronic structure, one of which allows the study of the molecular mechanics structure. The Born-Oppenheimer surface is then a multidimensional surface describing the energy of the molecule in terms of nuclear positions.
Molecular mechanics tends to be one of the standard methods of structural chemistry.
This function is presumed to have a minimum corresponding to stable equilibrium geometries. Bonds have 'Natural lengths and angles. As in a diagonal valence force field, one can then assume a harmonic potential with Hooke's law functions for bond stretching and bending. All forces are defined in terms of potential E Functions of the internal coordinates of the molecules that constitute the molecular force field. Inthese expressions, the sums extend over all bonds, angles, torsions, and nonbonded interactions between all atoms not bound to each other or to a common atom.