Incorporating charge transfer effects into a metallic empirical potential for accurate structure determination in (ZnMg)N nanoalloys

Abstract

We report the results of a combined empirical potential-density functional theory (EP-DFT) study to assess the global minimum structures of free-standing zinc–magnesium nanoalloys of equiatomic composition and with up to 50 atoms. Within this approach, the approximate potential energy surface generated by an empirical potential is first sampled with unbiased basin hopping simulations, and then a selection of the isomers so identified is re-optimized at a first-principles DFT level. Bader charges calculated in a previous work [A. Lebon, A. Aguado and A. Vega, Corros. Sci., 2017, 124, 35–45] revealed a significant transfer of electrons from Mg to Zn atoms in these nanoalloys; so the main novelty in the present work is the development of an improved EP, termed Coulomb-corrected-Gupta potential, which incorporates an explicit charge-transfer correction term onto a metallic Gupta potential description. The Coulomb correction has a many-body character and is fed with parameterized values of the ab initio Bader charges. The potentials are fitted to a large training set containing DFT values of cluster energies and atomic forces, and the DFT results are used as benchmark data to assess the performance of Gupta and Coulomb-corrected-Gupta EP models. Quite surprisingly, the charge-transfer correction is found to represent only 6% of the nanoalloy binding energies, yet this quantitatively small correction has a sizable beneficial effect on the predicted relative energies of homotops. Zn–Mg bulk alloys are used as the sacrificial material in corrosion-protective coatings, and the long-term goal of our research is to disclose whether those corrosion-protected capabilities are enhanced at the nanoscale.