Mass-weighted intrinsic coordinates

The MEP is defined as the path of steepest descent in mass-weighted Cartesian coordinates. This is also called intrinsic reaction coordinate (IRC). In reality, we know that many other paths close to the IRC path would also lead to a reaction and the percentage of the time each path is taken could be described by the Boltzmann distribution. 

The geometrical properties of the PES in the vicinity of a transition state mean that the steepest descent path down from the transition state (also generally calculated in mass-weighted coordinates, and called the intrinsic reaction coordinate or IRC) will usually lead only to a single reactant in one direction and a single product (or intermediate) in the other. However, a transition state can sometimes be shared by more than one reactant and/or product. One of these cases arises when the PES possesses a so-called valley-ridge inflection point (VRI). °... 

The choice of reaction path definition used as the reference for such a constrained dynamics is arbitrary any path may be used in practice. However, a natural choice in order to ensure that the simulation moves along the bottom of the potential energy valley connecting reactants/products with TS is the intrinsic reaction path (IRP) of Fukui.46,47 IRP by definition goes along the bottom of such a valley. IRP simply corresponds to a steepest descent path in a mass-weighted coordinates ... 

The actual path mapped out by the MEP on the PES is dependent on coordinate system. However, changes in coordinate system do not alter the nature of the stationary points on the PES (i.e. minima, TSs, etc.). One coordinate system, mass-weighted Cartesian coordinates (see Section 10.2.3), is especially significant for reaction dynamics, and the MEP in this coordinate system is known as the intrinsic reaction coordinate (IRC) [162]. In this section, we use the terms MEP, IRC, steepest descent path, and reaction path synonymously. 

GAMESS The Schlegel and Baker methods are available in this ab initio code. The intrinsic reaction coordinate (IRC) in mass-weighted Cartesian coordinates can be calculated using several possible methods for integrating the IRC equations. 

The simplest chemical reactions correspond to overcoming a single reaction barrier on the way from reactants to products through a saddle point along the intrinsic reaction coordinate (IRC). The IRC corresponds to the steepest descent trajectory (in the mass-weighted coordinates) from the saddle point to configurations of reactants and products. 

Coriolis coupling (p. 906 and 912) critical points (p. 888) cross section (p. 901) curvature coupling (p. 906 and 914) cycloaddition reaction (p. 944) democratic coordinates (p. 898) diabatic and adiabatic states (p. 949) donating mode (p. 914) early and late reaction barriers (p. 895) electrophilic attack (p. 938) entrance and exit channels (p. 895) exo- and endothermic reactions (p. 909) femtosecond spectroscopy (p. 889) Franck-Condon factors (p. 962) intrinsic reaction coordinate (IRC) (p. 902) inverse Marcus region (p. 954) mass-weighted coordinates (p. 903)... 

Vibrational analyses were performed in order to characterize the optimized structures as local minima or transition states. All the calculated frequencies, the zero point, and the thermal energies correspond to harmonic oscillators. The calculations of the intrinsic reaction coordinates (IRCs) for each transition state with mass-weighted internal coordinates were performed in order to check whether the transition states under consideration connect the expected reactants and products [ 14]. 

Berry phase (p. 780) mass-weighted coordinates (p. 781) intrinsic reaction coordinate (IRC) (p. 781) trajectory-in-molasses (p. 782) reaction path Hamiltonian (p. 783) natural coordinates (p. 784) vibrationally adiabatic approximation (p.785)... 

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