The cleavage force F(z) as a function of the additional separation z from equilibrium planar spacing can be usefully discussed in three regimes: (i) the Hooke law region F(z) = Az for small z, (ii) near its maximum, and (iii) the large z limit. Region (i) can be tackled by phonon theory, with appropriate inclusion of electron exchange and correlation interactions. Regime (iii), where F(z) = Ξ±z -3 , is characterized by the electronic plasma frequency, which in turn embodies long-range electron-electron correlations, in a metal or in a semiconductor such as Si. Here, special attention will be focused on region (ii) for Si and its relation to bond-breaking and electron correlation. The connection between cleavage force F(z) and surface energy is also emphasized. Electronic properties for noble and transition metals such as Cu and Fe can often be usefully subsumed into so-called glue models of many-body-force fields. Use of such force fields will be illustrated by reference to mechanical and tribological properties of Fe, with some attention again given to surface energies, which in turn correlate with point defect properties in a metal like Cu. Finally, bond-breaking models are invoked to discuss extended defects, and in particular a solitary wave equation is exhibited that can embrace the emission of phonons by a propagating screw dislocation.