E_vs_r_scan calculation style

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

Introduction

The E_vs_r_scan calculation style calculation creates a plot of the cohesive energy vs interatomic spacing, \(r\), for a given atomic system. The system size is uniformly scaled (\(b/a\) and \(c/a\) ratios held fixed) and the energy is calculated at a number of sizes without relaxing the system. All box sizes corresponding to energy minima are identified.

This calculation was created as a quick method for scanning the phase space of a crystal structure with a given potential in order to identify starting guesses for further structure refinement calculations.

Version notes

  • 2018-07-09: Notebook added.

  • 2019-07-30: Description updated and small changes due to iprPy version.

  • 2020-05-22: Version 0.10 update - potentials now loaded from database.

  • 2020-09-22: Setup and parameter definitions streamlined.

  • 2022-03-11: Notebook updated to reflect version 0.11. r_a() function replaced by the atomman.System.r0() method.

Additional dependencies

Disclaimers

  • NIST disclaimers

  • The minima identified by this calculation do not guarantee that the associated crystal structure will be stable as no relaxation is performed by this calculation. Upon relaxation, the atomic positions and box dimensions may transform the system to a different structure.

  • It is possible that the calculation may miss an existing minima for a crystal structure if it is outside the range of \(r\) values scanned, or has \(b/a\), \(c/a\) values far from the ideal.

Method and Theory

An initial system (and corresponding unit cell system) is supplied. The \(r/a\) ratio is identified from the unit cell. The system is then uniformly scaled to all \(r_i\) values in the range to be explored and the energy for each is evaluated using LAMMPS and “run 0” command, i.e. no relaxations are performed.

In identifying energy minima along the curve, only the explored values are used without interpolation. In this way, the possible energy minima structures are identified for \(r_i\) where \(E(r_i) < E(r_{i-1})\) and \(E(r_i) < E(r_{i+1})\).