Global analytical potential hypersurface for large amplitude nuclear motion and reactions in methane II. Characteristic properties of the potential and comparison to other potentials and experimental information

The global analytical potential surface for the electronic ground state of methane developed in paper I is analyzed and discussed in detail. A new determination of the experimental potential surface for the CH chromophore in CHD3, obtained from more recently measured line positions and integrated absorption coefficients, is also reported. The complete, nine-dimensional calculation of the vibrational ground state by diffusion quantum Monte Carlo on the fully anharmonic potential surface allows the determination of r(e) = (1.086 +/- 0.002) Angstrom with a high level of certainty from comparison with experimental values of rotational constants for methane and isotopomers. Other results regarding properties of the anharmonic potential surface close to the equilibrium configuration are theoretical values for the vibrationally induced electric dipole moments in CH3D, CH2D2, and CHD3, which are obtained in conjunction with a nine-dimensional, vector-valued representation of the electric dipole moment in this molecule and agree well with the experimental data. It is shown that, if equilibrium geometry and harmonic force field are fixed to experimental values, the overtone spectrum of the CH chromophore in CHD3 can be described in an acceptable way (<\V-cal - V-obs\> approximate to 40 cm(-1) up to 18 000 cm(-1) (METPOT 3). The agreement can be improved to within 17 cm(-1) (METPOT 4), on the average, if the anharmonic part of the model potential is refined with data from the experimentally derived, three-dimensional CH chromophore potential surface from Lewerenz and Quack (J. Chem. Phys. 88). For this purpose, the analytical representation of the potential, mainly along the bending degrees of freedom, must be sufficiently flexible, as shown by the present calculations. The accuracy regarding the description of spectroscopic data pertaining to highly excited vibrational states and the global character of the proposed potential surface representation render it a powerful instrument for the theoretical treatment of chemical reaction dynamics. A relation to reaction kinetics can be established through calculation of the lowest adiabatic channel on the complete nine-dimensional potential hypersurface for methane using quasiadiabatic channel quantum Monte Carlo techniques. It is found that the behavior of this channel, corresponding approximately to an exponential interpolation with a parameter alpha approximate to 0.7-0.8 Angstrom(-1) in the adiabatic channel model, is consistent with empirical results obtained from experiment. Further refinements of the models are feasible and expected, when full dimensional calculations of the solution of the rovibrational Schrodinger equation will be performed.