1.10.1 Basics

Materials relevant to reactors are held together by electrons. An interatomic potential expresses energy in terms of atomic positions. the electronic and mag­netic degrees of freedom are integrated out.

Most empirical potentials are derived on the basis of some approximation to quantum mechanical energies. If they are subsequently used in MD of solids, then what is actually used are forces: the derivative of the energy. For near-harmonic solids, it is actually the second derivative of the energy that governs the behavior.

A dilemma: Does the primary term covering ener­getics also dominate the second derivative of the energy? To take an extreme example, an equation which calculates the energy of a solid exactly for all configurations to 0.1% is: E = mc2 : most of the energy is in the rest mass of the atoms. But this is patently useless for calculating condensed matter properties. We encounter the same problem in a less extreme form in metals: should we concentrate the energy gained in delocalizing the electrons to form the metal, or is treating perturbations around the metallic state more useful? In general, the issue is ‘What is the reference state.’ Most potentials implic­itly assume that the free atom is the reference state.