Reaction-Path Dynamics Calculations Using Integrated Methods. The CF 3 CH 3 + OH Hydrogen Abstraction Reaction J. Espinosa-Garcı ´a † Departamento de Quı ´mica Fı ´sica, UniVersidad de Extremadura, 06071 Badajoz, Spain ReceiVed: December 18, 2001; In Final Form: March 19, 2002 The titlte reaction is used as a test to analyze the performance of the integrated methods by describing the intrinsic reaction path and then calculating kinetic and dynamic information for reactions involving the breaking-forming of covalent bonds in large molecules. The integrated methods split the “complete” system into two parts or layers and apply different levels of theory to each, which is especially interesting for the treatment of large molecules. We located and characterized the stationary points (reactants, products, and saddle point), calculated the energy, gradient, and Hessian along the intrinsic reaction path, and then, with this information, calculated thermal rate constants for the temperature range 250-500 K, using variational transition-state theory and multidimensional tunneling effect. The integrated method used (IMOMO) reproduces the values of the high-level method, corrects the deficiencies of the low-level method, and represents a substantial saving in computational cost. Its success is related to the higher-level description of the “model” system or inner layer (CH 4 + OH, in this case), with the effect of the lower-level description of the outer layer being smaller. The analysis of the coupling between the reaction coordinate and normal modes along the reaction path showed that the vibrational excitation of the reactive C-H stretching mode can enhance the forward rate constant and that the H 2 O normal modes (stretching and bending) can appear vibrationally excited in the exit channel. Variational effects and tunneling were found to be important, a behavior already known for the “model” system. Although we used high ab initio electronic levels, our theoretical rate constants markedly underestimate the experimental values. This problem arises from only partially introducing the correlation energy and using incomplete basis sets, a general problem in computational chemistry, and it is not directly related to the integrated method used here. 1. Introduction Even today, the complete construction of the potential energy surface (PES) of a polyatomic system is a prohibitive task, and several alternatives have been proposed to solve this problem. An interesting and successful alternative is the “direct dynamics” approach, 1-3 which describes a chemical reaction by using electronic structure calculations (energies, gradients and Hes- sians) without the mediation of a PES fit.The method is especially powerful when combined with dynamics methods that require PES information only in the region of configuration space along the reaction path. The major limitation of this approach is its high computational cost, which obviously increases with the molecular size. One economical approach involves density functional theory (DFT). 4-8 While the ab initio electronic structure calculations (with electron correlation) scale, at least, as N, 5 where N is the number of basis functions, DFT calculations scale as N 3 , with the consequent computational saving. It is well-known that DFT calculations or hybrid DFT calculations that mix in some Hartree-Fock exchange yield reasonable geometries and vi- brational frequencies, 9 atomization energies, 10 and enthalpies of formation. 11 However, when breaking-forming bonds are involved in the transition-state zone, DFT fails to perform well, 12-20 and generally underestimates the barrier height by several kcal mol -1 . For instance, Proynov et al. 19 analyzed the performance of several DFT methods with the much studied H + H 2 system, and found that all DFT and hybrid DFT calculations underestimate the barrier height by several kcal mol -1 , with the popular B3LYP calculation giving an error of 4.1 kcal mol -1 . A more exhaustive study was performed by Lynch and Truhlar 20 on a set of 22 reactions. In general, they found that the DFT and hybrid DFT methods underestimate the barrier height by about 3.4 kcal mol -1 , with one method (MPW1K: modified Perdew-Wang 1-parameter-method for kinetics) parametrized by the authors 21 themselves, predicting the most accurate saddle-point geometries and a mean unsigned error of only 1.5 kcal mol -1 for either basis set analyzed. Note that for this same set of 22 reactions, the MP2 (second-order Møller-Plasset perturbation theory) ab initio level has a mean error of 5.8 kcal mol -1 , and the more expensive QCISD (quadratic configuration interaction with single and double excitations) ab initio level has a mean error of 3.5 kcal mol -1 , indicating the necessity to use highly correlated wave functions and large basis sets. The accuracy limitation of the DFT and hybrid DFT approaches dissuade one from using them for the reaction-path description. An interesting and economic group of alternatives for the problem of large molecules and high-level calculations are the integrated methods, which describe different parts of the large system with different theoretical approaches. The main goal is to reproduce the results of a high-level theoretical calculation for a large, “complete” system, by dividing it into two parts: a small “model” system (which is the most active site, where the breaking-forming bonds are involved), and the “rest” of the † E-mail: joaquin@unex.es. 5686 J. Phys. Chem. A 2002, 106, 5686-5696 10.1021/jp0145513 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/16/2002