Potential-energy surfaces for dynamics studies of large chemical reactions

Diego Troya, Department of Chemistry, Virginia Tech

Theoretical reaction-dynamics studies monitor the time evolution of the atoms during a chemical reaction, which enables one to decipher the reaction mechanism and possible ways to control the reaction. One of the main difficulties in performing theoretical studies of the dynamics of chemical reactions resides in the derivation of a potential-energy surface that accurately describes the forces acting on the atomic nuclei while the reaction is taking place. While algebraic potential-energy surface expressions can be easily derived for reactions involving a few atoms, there is not a clear strategy to obtain these analytic functions for polyatomic chemical reactions. Instead, the direct-dynamics approach, in which the forces acting on the nuclei are obtained directly from electronic-structure calculations while the nuclei are moving from reagents to products, is the preferred approach for large chemical reactions. A problem with direct dynamics is that routine studies require millions of energy-gradient calculations. This poses strict limitations in the type of method and basis set that one can use to obtain the forces acting on the nuclei during their passage from reagents to products, and therefore jeopardizes the accuracy of the dynamics studies. In this presentation, we will show that specific-reaction-parameters (SRP) semiempirical Hamiltonians are extremely fast electronic-structure methods that enable extensive and accurate reaction-dynamics calculations. The SRP Hamiltonians are obtained via reparametrization of the original Hamiltonians so that the semiempirical energies agree with high-quality ab initio energies throughout the potential-energy surface. The accuracy of this approach is demonstrated by comparing calculated dynamics properties (cross sections, rovibrational product energy and angular distributions) with experiment for various radical + alkane reactions, including O(³P)+CH₄ à OH+CH₃, F+CH₄ à HF+CH₃, and H+CH₄,C₂H₆ à H₂+CH₃,C₂H₅.

Oral