Publications

Abstract: Mapping reaction pathways on complex potential energy surfaces (PESs) and locating transition states (TSs) is often used for understanding chemical reaction mechanism(s). The nudged elastic band (NEB) method is widely used for this purpose, but it becomes computationally expensive for large systems due to the repeated evaluation of energies and forces. We present an active learning algorithm coupled with the nudged elastic band, AL-NEB, for efficient convergence to the TS. AL-NEB constructs a surrogate PES and actively selects training points in two phases: (a) Exploration-Exploitation and (b) Renunciation. Strategies have been introduced for making the algorithm efficient and stable. We show the efficacy of the algorithm on several 2D analytical potentials, HCN isomerization, keto-enol tautomerization, and high-dimensional heptamer island diffusion (up to 525 degrees of freedom). In all cases, AL-NEB locates the “exact” TS on the chosen model chemistry with an order-of-magnitude fewer force evaluations than the standard NEB, demonstrating its scalability and efficiency.

Abstract: In the last few decades, several novel algorithms have been designed for finding critical points on PES and the minimum energy paths connecting them. This has led to considerably improve our understanding of reaction mechanisms and kinetics of the underlying processes. These methods implicitly rely on computation of energy and forces on the PES, which are usually obtained by computationally demanding wave-function or density-function based ab initio methods. To mitigate the computational cost, efficient optimization algorithms are needed. Herein, we present two new first-order optimization algorithms: adaptively accelerated relaxation engine (AARE), an enhanced molecular dynamics (MD) scheme, and accelerated conjugate-gradient method (Acc-CG), an improved version of the traditional conjugate gradient (CG) algorithm. We show the efficacy of these algorithms for unconstrained optimization on 2D and 4D test functions. Additionally, we also show the efficacy of these algorithms for optimizing an elastic band of images to the minimum energy path on 2-dimensional analytical potentials, heptamer island transitions, the HCN/CNH isomerization reaction, and the keto-enol tautomerization reaction.