Quantum and classical studies of vibrational motion of CH5+ on a global potential energy surface obtained from a novel ab initio direct dynamics approach

We report a full dimensional, ab initio based potential energy surface for CH5+. The ab initio electronic energies and gradients are obtained in direct-dynamics calculations using second-order Moller-Plesset perturbation theory with the correlation consistent polarized valence triple zeta basis. The potential energy and the dipole moment surfaces are fit using novel procedures that ensure the full permutational symmetry of the system. The fitted potential energy surface is tested by comparing it against additional electronic energy calculations and by comparing normal mode frequencies at the three lowest-lying stationary points obtained from the fit against ab initio ones. Well-converged diffusion Monte Carlo zero-point energies, rotational constants, and projections along the CH and HH bond lengths and the tunneling coordinates are presented and compared with the corresponding harmonic oscillator and standard classical molecular dynamics ones. The delocalization of the wave function is analyzed through comparison of the CH5+ distributions with those obtained when all of the hydrogen atoms are replaced by H-2 and H-3. The classical dipole correlation function is examined as a function of the total energy. This provides a further probe of the delocalization of CH5+. (C) 2004 American Institute of Physics.