Methyl radical:: ab initio global potential surface, vibrational levels and partition function

High-level multi-reference, configuration interaction calculations are performed on the CH3 system. Based on these results, an analytical, global potential energy surface capable of describing the H-2+CH((2)Pi) -> H+CH2((XB1)-B-3) reaction is developed. Employing this surface, and a Jacobi coordinates representation of the nuclear motion, the Lanczos method is used to calculate the lowest vibrational states of the methyl radical, CH3(X(2)A(2)). The lowest energy levels of the triplet methylene, CH2((XB1)-B-3), are also obtained. The CH3 levels associated with excitation of the v(2) out-of-plane umbrella motion display a strong quartic anharmonicity effect, in agreement with earlier experimental and theoretical results. A two-dimensional model based on coupling the umbrella and in-plane symmetric C-H stretch (v(1)) modes accounts for the v(2) level patterns. The impact of anharmonicity on the vibrational partition function is assessed.