Translation–rotation internal coordinates with splicing (TRICS): A new approach to molecular geometry interpolation

We present TRICS (Translation–Rotation Internal Coordinates with Splicing), a novel internal-coordinate-based interpolation method for generating high-quality approximations to minimum energy pathways. Non-redundant internal coordinate (IC) systems can be used to generate chemically reasonable interpolated pathways between reactant and product structures, but it may fail for regions of configuration space where the IC system becomes numerically ill-defined. TRICS overcomes this limitation using multiple IC systems to generate separate interpolated paths that are iteratively spliced together into one continuous and smooth trajectory. We benchmarked the new method on 679 chemically diverse reactions, demonstrating excellent performance in generating interpolated pathways close to optimized transition-state structures. We further demonstrate the utility of TRICS for generating smooth and barrier-free interpolated pathways between critical points on excited-state potential energy surfaces. TRICS is available from the open-source geomeTRIC package.