Developing methods to machine-learn potentials with application to nitrogen

Computational studies of condensed matter phases by molecular dynamics are limited by the lack of accurate and efficient interatomic potentials. The high level theories, such as density functional theory (DFT), provide accurate potential energy surface description but lack required efficiency for large scale problems. On the other end of the spectrum are empirical potentials which are fast but often not accurate enough. The emergence of new machine learning methods for the development of interatomic potentials aim to bridge this gap. This thesis presents the development of machine learning library for interatomic potentials. The Ta-dah! software is capable of generating machine-learned potentials for mono- and multi-component systems. The library provides wide range of atomic local environment descriptors and its modular structure allows quick implementation of new ideas. The library is fully interfaced with LAMMPS molecular dynamics software. The standard use of Ta-dah! involves training with data generated from DFT packages such as VASP and CASTEP. It also incorporates a training method for learning interatomic potentials from high level quantum mechanical theories, such as coupled cluster. The method allows to harvest existing databases of high quality quantum chemistry calculations to build interatomic potentials based on methods which, in principle, can exceed that achievable by density functional theory. The library is deployed to develop efficient and accurate interatomic potentials to study various systems. The thesis highlights molecular dynamics calculations with a new potential for molecular nitrogen, based on quantum chemistry data. Phase-coexistence and free energy calculations with this potential are used to describe the melt curve and several different crystal phases. This enables calculation of the phase diagram up to 10 GPa. The potential is also applied in the to study of the proposed “Frenkel Line” in the subcritical and supercritical regions

From Atoms to Colloids: Does the Frenkel Line Exist in Discontinuous Potentials?

The Frenkel line has been proposed as a crossover in the fluid region of phase diagrams between a "non-rigid" and a "rigid" fluid. It is generally described as a crossover in the dynamical properties of a material, and as such has been described theoretically using a very different set of markers from those with which is it investigated experimentally. In this study, we have performed extensive calculations using two simple yet fundamentally different model systems: hard spheres and square well potentials. The former has only hardcore repulsion, while the latter also includes a simple model of attraction. We computed and analysed a series of physical properties used previously in simulations and experimental measurements, and discuss critically their correlations and validity as to being able to uniquely and coherently locate the Frenkel in discontinuous potentials

From atoms to colloids : does the Frenkel line exist in discontinuous potentials?

The Frenkel line has been proposed as a crossover in the fluid region of phase diagrams between a "non-rigid" and a "rigid" fluid. It is generally described as a crossover in the dynamical properties of a material, and as such has been described theoretically using a very different set of markers from those with which is it investigated experimentally. In this study, we have performed extensive calculations using two simple yet fundamentally different model systems: hard spheres and square well potentials. The former has only hardcore repulsion, while the latter also includes a simple model of attraction. We computed and analysed a series of physical properties used previously in simulations and experimental measurements, and discuss critically their correlations and validity as to being able to uniquely and coherently locate the Frenkel in discontinuous potentials