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From a microscopic view, various natural phenomena involving a chemical reaction are generally caused by a series of elementary step continuously occurring at several points in phase space. When paying attention to each elementary step, it could be expressed as transitions between two regions in phase space. Since reaction system in most cases make a long stay within a stable region, regardless of the speed of its transition (~ a few ps), that step itself must be a 'rare event' happening extremely rarely. In addition, dynamics and static properties of stereo-specificity and aggregated structure are induced by cumulative results of an enormous number of elementary step in complex chemical reactions occurring in a long time scale (>=μs). It is, therefore, absolutely impossible to directly simulate them by employing traditional molecular simulation methods Under the circumstances, in our laboratory, we have proposed and developed Red Moon method (the hybrid Monte Carlo (MC) / Molecular Dynamics (MD) reaction method) to realize the practical atomistic simulation for massive complex chemical reaction systems.

Red Moon Method (The Hybrid MC/MD Reaction Method)

Red Moon method (the hybrid MC/MD reaction method) is a practical 'atomistic' molecular simulation for the complex chemical reaction systems. In this method, Monte Carlo (MC) method is applied to drive such chemical reactions, etc., that are rare events, while the molecular motions (molecular translation, rotation and vibration) are directly treated by using MD. Namely, by stochastically promoting the various chemical reactions, etc., this approach can handle the chemical phenomena appearing with the time and size (>100 A and μs ~ ms) which cannot be realized in conventional ones. In fact, the system can reach a state of chemical equilibrium (or stationary state) due to the above driving mechanism which generates such rare events as chemical reactions within the practical computational time (Rare event-driving mechanism). The concept of Red Moon method (the hybrid MC/MD reaction method) is shown in Figure 1.

First, we execute MD simulation in the region R until (one or) some pairs of atoms meet the necessary conditions and select configuration state r. Next, one of the pairs is virtually reacted to generate a configuration state s, relaxing the whole system through short MD simulation. In this step, after computing the potential energy difference ΔU_rs (= U_s - U_r) between the reactant and product, the present reaction event is accepted (or rejected) according to the transition probability under the Metropolis scheme,

By repeating such a combined cycle (MC/MD cycle) consisting of a couple of MC and MD treatment, the present method can promote changes of state that is brought about by a chemical reaction, and achieve a long-time simulation of large-scale chemical reaction systems.

We call a group of hybrid MC/MD methods of this kind, Red Moon method, from the acronyms of "Rare event-driving methodology of necessity". We would like you to refer to the paper listed below to learn more about our reasoning for the naming.

2-Chlorobutane racemization in DMF solution

In the field of stereochemistry, it is known that 2-chlorobutane molecule is one of typical organic compounds having one chiral carbon and exists as two stereoisomers, i.e., (R)- and (S)-enantiomers. In an equilibrium state, 2-chlorobutane molecules in the organic solution constantly isomerize between the two enantiomers and its enantiomeric excess is almost zero (~0% e.e.), which means they are 50:50 mixture of (R)- and (S)-enantiomers (racemic mixture).