tailieunhanh - Ebook Computational chemistry: Part 2

Part 2 book “Computational chemistry” has contents: Reaction coordinates, reaction rates, potential energy surfaces, conformation searching, fixing self-consistent field convergence problems, solvation, electronic excited states, spin contamination, force field customization, and other contents. | Computational Chemistry: A Practical Guide for Applying Techniques to Real-World Problems. David C. Young Copyright ( 2001 John Wiley & Sons, Inc. ISBNs: 0-471-33368-9 (Hardback); 0-471-22065-5 (Electronic) 18 Reaction Coordinates In order to de®ne how the nuclei move as a reaction progresses from reactants to transition structure to products, one must choose a de®nition of how a reaction occurs. There are two such de®nitions in common use. One de®nition is the minimum energy path (MEP), which de®nes a reaction coordinate in which the absolute minimum amount of energy is necessary to reach each point on the coordinate. A second de®nition is a dynamical description of how molecules undergo intramolecular vibrational redistribution until the vibrational motion occurs in a direction that leads to a reaction. The MEP de®nition is an intuitive description of the reaction steps. The dynamical description more closely describes the true behavior molecules as seen with femtosecond spectroscopy. MINIMUM ENERGY PATH The MEP is de®ned 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. There are several algorithms for ®nding a MEP. The most reliable of these algorithms are the current generation of methods that start from the transition structure. Simply using a steepest-descent method does not give a good description of the MEP. This is because the points chosen by steepest-descent algorithms tend to oscillate around the reaction coordinate as shown in Figure . The algorithms incorporated in most software packages correct for this problem. The reaction coordinate is calculated in a number of steps. If too few steps are used, then the points that are computed will follow the reaction .