(234d) Improving the Initial Guess for a Nudged Elastic Band Calculation By Incorporating Chemical Intuition

Authors: 
Groden, K., Washington State University
McEwen, J. S., Washington State University
The Nudged Elastic Band (NEB) is a powerful technique incorporated into multiple computational chemistry packages for the elucidation of complex reaction pathways [1, 2]. Although modern computers are certainly capable of handling these calculations, their execution, as with all iterative mathematical programs, can be severely affected by the initial guesses provided by the user. Typically, the initial guesses for reaction pathways are constructed via linearly interpolating the coordinates of atoms from an initial state to a final state. Recently, Smidstrup et al. [3] proposed a method where bond distances between atoms were interpolated rather than coordinates and showed performance improvements to the respective NEB calculations using these improved initial guesses. However, as most calculations using the NEB method are for systems with more than seven atoms, bond distances cannot be perfectly interpolated along a reaction pathway due to a lack of degrees of freedom in the involved optimizations (more bonds than atomic coordinates to move). To this end, Smidstrup et al. weighted the bonds in their construction of initial guesses based on the inverse quartic bond length, which ensured that shorter were interpolated at the expense of longer ones.

As there may certainly be cases where shorter bonds may not require accurate interpolation and longer ones might, we chose to turn to a user’s chemical intuition to guide these improved interpolations. Our approach allows the user to specify which bonds, based on their knowledge of the underlying chemistry, should be considered important in the construction of an initial guess for a given reaction pathway, which are implemented in an objective function for an NEB-style minimization of the reaction pathway. Bonds not selected to be important are ignored in the aforementioned objective function and therefore the associated atoms only move in accordance to the artificial spring forces present during the NEB minimization. Further, the specific computational implementation allows for the user to specify how they would like bonds to change throughout the reaction pathway (not strictly limited to pure linear interpolation). Here, we present the performance of our method for constructing initial guesses to that of Smidstrup and strict coordinate linear interpolation for a variety of reaction systems through analysis of the ionic steps required for the subsequent quantum mechanical NEB calculations. We also present a specific case where initial guesses generated through our approach proved able to adequately map a reaction pathway while coordinate linear interpolation for an initial guess gave unrealistic results [4].

References:

[1] G. Henkelman, H. Jónsson, Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points, The Journal of Chemical Physics, 113 (2000) 9978-9985.

[2] G. Henkelman, B.P. Uberuaga, H. Jónsson, A climbing image nudged elastic band method for finding saddle points and minimum energy paths, The Journal of Chemical Physics, 113 (2000) 9901-9904.

[3] S. Smidstrup, A. Pedersen, K. Stokbro, H. Jónsson, Improved initial guess for minimum energy path calculations, The Journal of Chemical Physics, 140 (2014) 214106.

[4] A.J.R. Hensley, J. Zhang, I. Vinçon, X.P. Hernandez, D. Tranca, G. Seifert, J.-S. McEwen, Y. Wang, Mechanistic understanding of methanol carbonylation: Interfacing homogeneous and heterogeneous catalysis via carbon supported Ir-La, Journal of Catalysis, 361 (2018) 414-422.