A new algorithm for efficient direct dynamics calculations of large-curvature tunneling and its application to radical reactions with 9-15 atoms

We present a new algorithm for carrying out large-curvature tunneling calculations that account for extreme corner-cutting tunneling in hydrogen atom, proton, and hydride transfer reactions. The algorithm is based on two-dimensional interpolation in a physically motived set of variables that span the space of tunneling paths and tunneling energies. With this new algorithm, we are able to carry out density functional theory direct dynamics calculations of the rate constants, including multidimensional tunneling, for a set of hydrogen atom transfer reactions involving 9-15 atoms and up to 7 nonhydrogenic atoms. The reactions considered involve the abstraction of a hydrogen atom from hydrocarbons by a trifluoromethyl radical, and in particular, we consider the reactions of CF₃ with CH₄, C₂H₆, and C₃H₈. We also calculate several kinetic isotope effects. The electronic structure is treated by the MPWB1K/6-31+G(d,p) method, which is validated by comparison to experimental results and to CBS-Q, MCG3, and G3SX(MP3) calculations for CF₃ + CH₄. Harmonic vibrational frequencies along the reaction path are calculated in curvilinear coordinates with scaled frequencies, and anharmonicity is included in the lowest-frequency torsion.