“On-the-fly” Crystal: How to reliably and automatically characterize and construct potential energy surfaces

In this work, the Crystal code, developed previously by the authors to find “holes” as well as legitimate transition states in existing potential energy surface (PES) functions [JPC Lett. 11, 6468 (2020)], is retooled to perform on-the-fly “direct dynamics”-type PES explorations, as well as automatic construction of new PES functions. In all of these contexts, the chief advantage of Crystal over other methods is its ability to globally map the PES, thereby determining the most relevant regions of configuration space quickly and reliably—even when the dimensionality is rather large. Here, Crystal is used to generate a uniformly-spaced grid of density functional theory (DFT) or ab initio points, truncated over the relevant regions, which can then be used to either: (a) hone in precisely on PES features such as minima and transition states, or; (b) create a new PES function automatically, via interpolation. Proof of concept is demonstrated via application to three molecular systems: water (H2O), methane (CH4), and methylene imine (CH2NH).