Global reaction route mapping (GRRM), a fully-automated search for all important reaction pathways relevant to a given purpose, on the basis of quantum chemical calculations enables systematic elucidation of complex chemical reaction mechanisms. However, GRRM had previously been limited to very simple systems. This is mainly because such calculations are highly demanding even in small systems when a brute-force sampling is considered. Hence, we have developed two independent but complementary methods: anharmonic downward distortion following (ADDF) and artificial force induced reaction (AFIR) methods. ADDF can follow reaction pathways starting from local minima on the potential energy surface (PES) toward transition structures (TSs) and dissociation channels. AFIR can find pathways starting from two or more reactants toward TSs for their associative reactions. In other words, ADDF searches for A → X type isomerization and A → X + Y type dissociation pathways, whereas AFIR finds A + B → X (+ Y) type associative pathways. Both follow special paths called the ADDF path and the AFIR path, and these tend to pass through near TSs of corresponding reaction pathways, giving approximate TSs. Such approximate TSs can easily be re-optimized to corresponding true TSs by standard geometry optimizations. On the basis of these two methods, we have proposed practical strategies of GRRM. The GRRM strategies have been applied to a variety of chemical systems ranging from thermal- and photochemical-reactions in small systems to organometallic- and enzyme-catalysis, on the basis of quantum chemical calculations. In this perspective, we present an overview of the GRRM strategies and some results of applications. Their practical usage for systematic prediction is also discussed.

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Technical details of a new global mapping technique for finding equilibrium (EQ) and transition structures (TS) on potential energy surfaces (PES), the scaled hypersphere search (SHS) method (Ohno, K.; Maeda, S. Chem. Phys. Lett. 2004, 384, 277), are presented. On the basis of a simple principle that reaction pathways are found as anharmonic downward distortions of PES around an EQ point, the reaction pathways can be obtained as energy minima on the scaled hypersphere surface, which would have a constant energy when the potentials are harmonic. Connections of SHS paths between each EQ are very similar to corresponding intrinsic reaction coordinate (IRC) connections. The energy maximum along the SHS path reaches a region in close proximity to the TS of the reaction pathway, and the subsequent geometry optimization from the SHS maximum structure easily converges to the TS. The SHS method, using the one-after-another algorithm connecting EQ and TS, considerably reduces the multidimensional space to be searched to certain limited regions around the pathways connecting each EQ with the neighboring TS. Applications of the SHS method have been made to ab initio surfaces of formaldehyde and propyne molecules to obtain systematically five EQ and nine TS for formaldehyde and seven EQ and 32 TS for propyne.

Global reaction route mapping of equilibrium structures, transition structures, and their connections on potential energy surface (PES) has been done for MCHO (M = H, Li, Na, Al, Cu) and HCO2M (M = H, Li). A one-after-another technique based on the scaled hypersphere search method has been successfully applied to exploring unknown chemical structures, transition structures, and reaction pathways for organometallic systems. Upon metal substitution, considerable changes of stable structures, reaction pathways, and relative heights of transition structures have been discovered, though some features are similar among the analogues. Al and Cu atoms were found to behave as very strong scissors to cut the CO double bond in MCHO. Energy profiles of the CO insertion into Li-H and Li-CH3 bonds were found to be very similar, especially around the structures where the Li atom is not directly connected with the methyl group, which indicates little effects of alkyl substitution on the reaction route topology.

The intrinsic reaction coordinate (IRC) approach has been used extensively in quantum chemical analysis and prediction of the mechanism of chemical reactions. The IRC gives a unique connection from a given transition structure to local minima of the reactant and product sides. This allows for easy understanding of complicated multistep mechanisms as a set of simple elementary reaction steps. In this article, three topics concerning the IRC approach are discussed. In the first topic, the first ab initio study of the IRC and a recent development of an IRC calculation algorithm for enzyme reactions are introduced. In the second topic, cases are presented in which dynamical trajectories bifurcate and corresponding IRC connections can be inaccurate. In the third topic, a recent development of an automated reaction path search method and its application to systematic construction of IRC networks are described. Finally, combining these three topics, future perspectives are discussed.

In these five decades, many useful tools have been developed for exploring quantum chemical potential energy surfaces. The success in theoretical studies of chemical reaction mechanisms has been greatly supported by these tools. However, systematic prediction of reaction mechanisms starting only from given reactants and catalysts is still very difficult. Toward this goal, we describe the artificial force induced reaction (AFIR) method for automatically finding reaction paths of type A + B → X (+ Y). By imposing an artificial force to given reactants and catalysts, the method can find the reactive sites very efficiently. Further pressing by the artificial force provides approximate transition states and product structures, which can be easily reoptimized to the corresponding true ones. This procedure can be executed very efficiently just by minimizing a single function called the AFIR function. All important reaction paths can be found by repeating this cycle starting from many initial orientations. We also discuss perspectives of automated reaction path search methods toward the above goal.

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This article reports implementation and performance of the artificial force induced reaction (AFIR) method in the upcoming 2017 version of GRRM program (GRRM17). The AFIR method, which is one of automated reaction path search methods, induces geometrical deformations in a system by pushing or pulling fragments defined in the system by an artificial force. In GRRM17, three different algorithms, that is, multicomponent algorithm (MC‐AFIR), single‐component algorithm (SC‐AFIR), and double‐sphere algorithm (DS‐AFIR), are available, where the MC‐AFIR was the only algorithm which has been available in the previous 2014 version. The MC‐AFIR does automated sampling of reaction pathways between two or more reactant molecules. The SC‐AFIR performs automated generation of global or semiglobal reaction path network. The DS‐AFIR finds a single path between given two structures. Exploration of minimum energy structures within the hypersurface in which two different electronic states degenerate, and an interface with the quantum mechanics/molecular mechanics method, are also described. A code termed SAFIRE will also be available, as a visualization software for complicated reaction path networks. © 2017 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.