Quantum dynamics of the H+CH4→H2+CH3 reaction in curvilinear coordinates: Full-dimensional and reduced dimensional calculations of reaction rates

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.