THE DETERMINATION OF INTRINSIC REACTION COORDINATES BY DENSITY-FUNCTIONAL THEORY

The first implementation of the intrinsic reaction coordinate (IRC) method within the density functional theory (DFT) framework is presented. The implementation has been applied to four different types of chemical reactions represented by the isomerization process, HCN = HNC (A); the S(N)2 process, H- + CH4 = CH4 + H- (B); the exchange process, H. + HX = HX + H. (X = F, Cl) (C); and the elimination process, C2H5Cl = C2H4 + HCl (D). The present study presents for each process optimized structures and calculated harmonic vibrational frequencies for the reactant(s), the transition state, and the product(s) along with the IRC path connecting the stationary points. The calculations were carried out within the local density approximation (LDA) as well as the LDA/NL scheme where the LDA energy expression is augmented by Perdew's and Becke's nonlocal (NL) corrections. The LDA and LDA/NL results are compared with each other as well as the best available ab initio calculations and experimental data. For reaction (D), ab initio calculations based on MP2 geometries and MP4SDTQ energies have been added due to the lack of accurate published post-HF calculations on this process. A detailed discussion is provided on the efficiency of the IRC algorithms, the relative accuracy of the DFT and ab initio schemes, as well as the reaction mechanisms of the four reactions. It is concluded that the LDA/NL scheme affords the same accuracy as does the MP4 method. The post-HF methods seem to overestimate activation energies, whereas the corresponding LDA/NL estimates are too low. The LDA activation energies are even lower than the LDA/NL counterparts. The incorporation of the IRC method into the DFT framework provides a promising and reliable tool for probing the chemical reaction path on the potential energy surfaces, even for large-size systems. IRC calculations by ab initio methods of an accuracy similar to the LDA/NL scheme, such as the MP4 scheme, are not feasible. (C) 1994 John Wiley & Sons, Inc.