The ReaxFF polarizable reactive force fields for molecular dynamics simulation of ferroelectrics

Abstract

We use ab initio Quantum Mechanical (QM) calculations to derive a force field that accurately describes the atomic interactions in BaTiO3 allowing, via Molecular Dynamics (MD), the simulation of thousands of atoms. A key feature of the force field (denoted ReaxFF) is that charge transfer and atomic polarization are treated self-consistently. The charge on each atom is separated into a core, described as a Gaussian distribution with fixed total charge (e.g. +4 for Ti), and a valence charge, also described as a Gaussian distribution. The valence charges can flow in response to its environment as described via Charge Equilibration (QEq). The restoring force between a core and its valence electrons is given be the electrostatic interaction between the two charge distributions. Thus each atom has four universal parameters describing the electrostatics which are determined once from fitting to the QM charge distributions on a representative set of finite clusters. The nonelectrostatic interactions (Pauli repulsion, dispersion) are described with a Morse potential, leading to 3 additional universal parameters for each pair of atoms. We optimized the Morse parameters to reproduce the zero temperature Equation of State (energy- and pressure-volume curves) obtained using QM methods of cubic and tetragonal BaTiO_3 over a wide pressure range. We then use the ReaxFF with MD to study thermal properties of BaTiO_3, in particular the cubic to tetragonal phase transition. Our MD simulations indicate that the transition temperature obtained using ReaxFF is in good agreement with experiment.