BENCHMARK CALCULATIONS WITH CORRELATED MOLECULAR WAVE-FUNCTIONS .11. ENERGETICS OF THE ELEMENTARY REACTIONS F-2, O+H-2, AND H'+HCL(H)

An explicit treatment of electron correlation is required to predict a ccurate energetics, barrier heights, and saddle point geometries for c hemical reactions. Several theoretical methods for treating electron c orrelation (multireference configuration interaction, perturbation the ory, and coupled cluster methods) have been thoroughly evaluated for t he F(P-2) + H-2(X-1 Sigma(g)(+)) and O(P-3) + H-2(X-1 Sigma(g)(+)) abs traction reactions as well as for the H'(2S) + HCl(X-1 Sigma(+)) excha nge reaction using correlation consistent basis sets. The basis set de pendence of the reaction energy defects, barrier heights, and saddle p oint geometries have been determined for each theoretical method. Addi tion of diffuse functions to the basis set (aug-cc-pVnZ) was found to substantially increase the convergence rate. Calculations with the lar gest basis set (aug-cc-pV5Z) allowed an unambiguous comparison of the relative performance of each correlation method. For each reaction, th e R-UCCSD(T) results closely parallel the most accurate MRCI results a nd are in good agreement with experiment. In contrast, unrestricted pe rturbation theory methods predict barriers that are too large by 2.7-4 .4 kcal/mol (MP2), 3.5-4.2 kcal/mol (MP3), and 1.3-1.7 kcal/mol (MP4).