www.asiachem.news December 2021 | 57 DFT-BASED REACTION mechanism study is currently popular. By performing DFT calculations for each ever-changing nuclear configuration, molecular dynamics (MD) calculations simulate the motion of atoms while solving Newton’s eqation of motion based on the gradient of the PES.2 Nevertheless, MD calculations require DFT calculations every 10-15 seconds in the simulation step width, and as many as 106 DFT calculations are necessary to run a simulation of 10-9 seconds. Many reaction mechanism studies discuss energy profiles.3-5 After geometry optimization obtains the transition state (TS) of each elementary reaction process,6 the movement of atoms during the reaction is visualized by the intrinsic reaction coordinate (IRC) calculated from the TS.7,8 Then, an energy profile can be drawn using the energy levels of the TS and the two end-points along the IRC path. Since several tens to several hundreds of DFT calculations are needed for one geometry optimization and one IRC calculation each, the calculation cost is much lower than MD calculations. On the other hand, the TS obtained by geometry optimization depends on the initial structure, which is generally created based on the computators’ own experience and intuition. Therefore, it is necessary to repeat DFT-based geometry optimization calculations for the mechanism assumed by the computators until the energy profile becomes consistent with the experimental result. Recently, several automated reaction path search methods that do not require the initial structure of TS have been developed.9-11 The methods enable systematic analysis of reaction mechanisms without relying on the computators’ experience or intuition. In addition, the automated reaction path search methods are opening the way to ab initio prediction of new chemical reactions passing through unknown reaction paths. This article outlines the artificial force induced reaction (AFIR) method, which has been developed by the authors.12-15 Further, by applying the rate constant matrix contraction (RCMC) method, a kinetic analysis method developed by the authors, to the reaction path network obtained by the AFIR method, the products and their formation paths can be elucidated.16,17 Combined usage of the AFIR and RCMC methods also facilitates on-thefly kinetic simulation to explore elementary reaction processes while solving the chemical kinetics.18 Finally, this article introduces quantum chemistry aided retrosynthetic analysis (QCaRA), which searches reaction paths backward from a product to possible reactants, and its application to chemical reaction discovery.19 Artificial Force Induced Reaction (AFIR) Method The basic idea behind the AFIR method is quite simple, as explained below. The fragments in a molecule or complex are repeatedly pushed against each other or pulled apart by an artificial force. Fig. 1(a) shows the AFIR function FAFIR applying an artificial force between fragments A and B, where Q is a set of variables describing the molecular structure, E is PES, α is a parameter defining the strength of the artificial force, Ri is the covalent radius of the i-th atom, rij is the distance between the i-th and j-th atoms, and p is a parameter (after adjustment, it is set p = 6). This procedure for minimizing FAFIR corresponds to the application of an artificial force between substructures A and B. The three examples in Fig. 1(b) indicate that different molecules can be obtained by specifying various fragments, A and B, in each case and minimizing FAFIR. In other words, by systematically generating combinations of A and B and minimizing FAFIR for each instance, new

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