MRCI calculations of the lowest potential energy surface for CH ₃OH and direct ab initio dynamics simulations of the O( ¹D) + CH ₄ reaction

The stationary point geometries and frequencies on the lowest singlet potential energy surface for the CH ₃OH system are calculated using the complete-active-space self-consistent-field method. The energetics are refined using a restricted internally contracted multireference configuration interaction (MRCI) method at the complete basis set (CBS) limit. The CBS energy is extrapolated using the scheme of Halkier et al. with two large basis sets: aug-cc-pVDZ and aug-cc-pVTZ. The implications of our calculated results concerning the O( ¹D) + CH ₄ and OH + CH ₃ reactions are discussed. In addition, the O( ¹D) + CH ₄ reaction at a collision energy of 6.8 kcal/mol is investigated using a variant of the "scaling all correlation" (SAC) method of Truhlar et al. and the coupled-cluster double-excitation (CCD) method in a direct dynamics study with a D95(d,p) basis set. The results show that the O( ¹D) + CH ₄ → OH + CH ₃ reaction occurs both via direct and long-lived intermediate pathways. The differential cross section for the direct reaction to form OH is forward peaked with a nearly Isotropic background. Finally, the branching fractions for OH, H, H ₂, and H ₂O are predicted to be 0.725:0.186:0.025:0.064.