Quantum dynamics of the H+CH_4→H_2+CH_3 reaction in curvilinear coordinates: Full-dimensional and reduced dimensional calculations of reaction rates

摘要

Full-dimensional quantum dynamics calculations for the H+ CH_4→H_2+ CH_3 reaction using curvilinear coordinates are presented. A curvilinear coordinate system to describe reactions of the type X+ YCH_3→XY+CH_3 is developed which facilitates efficient calculations using the multiconfigurational time-dependent Hartree (MCTDH) approach. To describe the bending motion of the X and Y atoms relative to the axis defined by the CH_3 fragment, coordinates based on stereographic projection are introduced. These coordinates yield a kinetic energy operator free of singularities within the dynamically relevant region. Employing this curvilinear coordinate system, full-dimensional and reduced dimensional MCTDH calculations study the cumulative reaction probability (for J=0) and the thermal rate constant for the H+CH_4 reaction on the Jordan-Gilbert potential energy surface [J. Chem. Phys. 102, 5669 (1995)]. The full-dimensional results agree very well with previous full-dimensional MCTDH results which used transition state based normal coordinates. The results of our eight-dimensional (8D) calculations are in reasonable agreement with the full-dimensional ones. They deviate significantly from older 8D results of Zhang et al. [J. Chem. Phys. 127, 234213 (2007)] but agree well with more recent results from the same group