Exploring, fitting, and characterizing the configuration space of materials with multiscale universal descriptors

Descriptors of the environment of an atom in a material give a similarity metric between different structures. We present a universal set of multiscale Smooth Overlaps of Atomic Position (SOAP) parameters that can be used for a wide range of purposes. For each atom type these consist of two or more SOAP expansions of the smoothed atomic neighbour density with cutoffs (and proportionately scaled smoothness parameters) related by a constant factor, covering bond lengths from the shortest to the longest expected for that species in the system. These descriptors can be used as part of an automatic interatomic potential generation process by combining iterated random structure search (RSS) with Gaussian Approximation Potential (GAP) fitting. We show results from the GAP-RSS process for elemental and multicomponent systems, as the procedure simultaneously explores and fits a wide range of structures. The descriptors can also be used in the context of characterization of large sets of configurations. As an example, we show the results of large scale and long time simulations such as quenching of a liquid into an amorphous structure, and the relation between the fitting data set of a potential and the configurations that occur in tests of physically meaningful material properties.