ReaxFF_MgH Reactive Force Field for Magnesium Hydride Systems

Abstract

We have developed a reactive force field (ReaxFF MgH ) for magnesium and magnesium hydride systems. The parameters for this force field were derived from fitting to quantum chemical (QM) data on magnesium clusters and on the equations of states for condensed phases of magnesium metal and magnesium hydride crystal. The force field reproduces the QM-derived cell parameters, density, and the equations of state for various pure Mg and MgH 2 crystal phases as well as and bond dissociation, angle bending, charge distribution, and reaction energy data for small magnesium hydride clusters. To demonstrate one application of ReaxFF MgH , we have carried out MD simulations on the hydrogen absorption/desorption process in magnesium hydrides, focusing particularly on the size effect of MgH 2 nanoparticles on H 2 desorption kinetics. Our results show a clear relationship between grain size and heat of formation of MgH 2 ; as the particle size decreases, the heat of formation increases. Between 0.6 and 2.0 nm, the heat of formation ranges from -16 to -19 kcal/Mg and diverges toward that of the bulk value (-20.00 kcal/Mg) as the particle diameter increases beyond 2 nm. Therefore, it is not surprising to find that Mg nanoparticles formed by ball milling (20-100 nm) do not exhibit any significant change in thermochemical properties.

The ReaxFF reactive force-field: development, applications and future directions

“…Since 2005, the ReaxFF functional form has been stable, although optional additions, such as angular terms to destabilise Mg-Mg-H zero-degree angles 6 or double-well angular terms necessary for describing aqueous transition metal ions, 7 have occasionally been added to the potential. Goddard and coworkers implemented an additional attractive van der Waals term to improve performance for nitramine crystals (ReaxFF-lg).…”

A molecular model for carbon black primary particles with internal nanoporosity

“…Also neglecting covalent bonds formed between graphite units on the edges could potentially underestimate the stability of carbon black primary particle [45]. In general, a non-reactive force field is a more effective alternative to simulate C-C bond formation [46]. The C-C bond formation is particularly important to realistically represent the molecular structure of porous particles that are created from nonporous carbon particles.…”

Modeling the sorption dynamics of NaH using a reactive force field

“…6 A key feature in ReaxFF is using the bond-order formalism that allows for bond breaking and formation as per Tersoff, 7 Brenner, 8 and environment dependent interatomic potential 9 approach. ReaxFF includes polarizable charges calculated using electronegativity equalization method ͑EEM͒, 10 which provides a ge-ometry dependent charge distribution.…”