1.10.13.1 Useful Concepts Without True Physical Meaning

For very large simulations, imaging is a problem, since showing all atoms in a massive simulation is likely to obscure the important regions. There are a number of heuristic quantities arising from the interatomic potential which can be used to pick out the atoms associated with atypical configurations of interest.

• Most empirical potentials define the energy per atom.

• The atomic level stress55

“b 1 a b a b

s = 2O ~ m, v’vP’

1 j

where f is the force on atom i due to atom j which determines the glass transition.56

• The concept of ‘local crystal structure’ can be used to locate twin boundaries, phase transitions, etc. This may be done by common neighbor analysis, or bespoke investigation of pair and angular distribution functions to search for particular configurations.57

• The balance between pair and many-body ener­gies in glue-type models, or more significantly the various angular density functions of MEAM.

• The magnetization in ‘magnetic’ potentials.

Although uniquely defined for a given potential, many of these are not well-defined concepts in electronic structure. Yet, they can be extremely useful in identi­fying the atoms in far-from-equilibrium environments.

1.10.9 Summary

Interatomic potential development is a continuing challenge for materials modeling. They represent the only way to perform MD, which in turn is crucial for the nonequilibrium and off-lattice processes, which dominate radiation damage. Despite best efforts, few potentials can be reliably employed to predict quanti­tative energies beyond where they are fitted. Their most useful role is to reveal processes and topologies that might be of importance in real materials.