Deconstructing the ONIOM Hessian: Investigating Method Combinations for Transition Structures.

Abstract

Developments in biochemistry and materials sciences have led to increasing interest in the reactivity of large chemical systems, presenting theoretical and computational challenges that can be addressed with hybrid methods such as ONIOM. Here, we show that the diagonalized ONIOM Hessian can be partitioned/deconstructed into contributions from the individual subcalculations-indicating the curvature of their potential energy surfaces (PESs)-without increasing the computational cost. The resulting pseudofrequencies have particular application in the study of transition structures and higher-order saddle points with ONIOM, where we find that an imaginary frequency may result from combining subcalculations for which the corresponding vibrational frequencies are all real. Two cycloaddition reactions, including functionalization of a 150 atom (5,5) single-walled carbon nanotube, demonstrate how this analysis of pseudofrequencies allows identification of critical points where further exploratory work should be carried out to ensure that the ONIOM PES correctly approximates the target.