isolated_atom calculation style

Lucas M. Hale, lucas.hale@nist.gov, Materials Science and Engineering Division, NIST.

Introduction

The isolated_atom calculation style evaluates the base energies of all atomic models associated with an interatomic potential. For some potentials, the isolated energy values are necessary to properly compute the cohesive energy of crystal structures. This also provides a simple test whether a potential implementation is compatible with a version of LAMMPS.

Version notes

  • 2020-09-22: Notebook first added.

  • 2022-03-11: Notebook updated to reflect version 0.11.

Additional dependencies

Disclaimers

  • NIST disclaimers

  • Some potentials have two cutoffs with atomic energies outside the first being the “isolated” energy while outside the second have zero energy. The first isolated energy values for those potentials can be found using the diatom_scan calculation instead.

Method and Theory

The calculation loops over all symbol models of the potential and creates a system with a single particle inside a system with non-periodic boundary conditions. The potential energy of each unique isolated atom is evaluated without relaxation/integration.

The cohesive energy, \(E_{coh}\), of a crystal structure is given as the per-atom potential energy of the crystal structure at equilibrium \(E_{crystal}/N\) relative to the potential energy of the same atoms infinitely far apart, \(E_i^{\infty}\)

\[E_{coh} = \frac{E_{crystal} - \sum{N_i E_{i}^{\infty}}}{N},\]

Where the \(N_i\) values are the number of each species \(i\) and \(\sum{N_i} = N\).

For most potentials, \(E_i^{\infty}=0\) meaning that the measured potential energy directly corresponds to the cohesive energy. However, this is not the case for all potentials as some have offsets either due to model artifacts or because it allowed for a better fitted model.