THE DETERMINATION OF INTRINSIC REACTION COORDINATES BY DENSITY-FUNCTIONAL THEORY

The first implementation of the intrinsic reaction coordinate (IRC) me thod within the density functional theory (DFT) framework is presented . The implementation has been applied to four different types of chemi cal 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 rea ctant(s), the transition state, and the product(s) along with the IRC path connecting the stationary points. The calculations were carried o ut within the local density approximation (LDA) as well as the LDA/NL scheme where the LDA energy expression is augmented by Perdew's and Be cke's nonlocal (NL) corrections. The LDA and LDA/NL results are compar ed with each other as well as the best available ab initio calculation s and experimental data. For reaction (D), ab initio calculations base d on MP2 geometries and MP4SDTQ energies have been added due to the la ck of accurate published post-HF calculations on this process. A detai led discussion is provided on the efficiency of the IRC algorithms, th e relative accuracy of the DFT and ab initio schemes, as well as the r eaction 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 corre sponding 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 surface s, 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, a re not feasible. (C) 1994 John Wiley & Sons, Inc.