Energetic and structural features of the CH4+O(P-3)-> CH3+OH abstraction reaction: Does perturbation theory from a multiconfiguration reference state(finally) provide a balanced treatment of transition states?

The stationary points of the CH4+O(P-3)--> CH3+OH abstraction reaction have been identified at the fully optimized reaction space (FORS) level. For th ree sets of geometries (FORS plus unrestricted and restricted-open-shell Mo ller-Plesset second order perturbation theory), single-point calculations b y unrestricted Moller-Plesset fourth order perturbation theory (UMP4), by u nrestricted coupled cluster theory with single and double excitations and a quasiperturbative treatment of fourth- and fifth-order triple-excitation t erms (CCDS(T)), and by multireference Moller-Plesset second order perturbat ion theory (MRMP2) were also performed for the classical barrier height and energy of the reaction. Calculations carried out at the MRMP2/cc-pVTZ//FOR S/cc-pVTZ level predict values for the forward vibrationally adiabatic barr ier height and for the energy of the reaction at 0 K equal to 10.3 and 2.0 kcal/mol, respectively. This is in excellent agreement with experiments tha t show values of the activation energies in the range of 9-12 kcal/mol (at temperatures below 1500 K) and an energy of reaction equal to 1.8 kcal/mol. Expectation values of (S) over cap(2), where (S) over cap is total electro n spin, and also the values the coefficients of the configuration state fun ctions show that the reactants and the products of this reaction are well d escribed by single-configuration reference states but that the transition s tructure has a much higher multiconfigurational character. We conclude that MRMP2 may provide some light at the end of the tunnel in the long-standing quest for method that includes nondynamical and dynamical correlation in a balanced way in the electronic wave function of open-shell transition stat es. (C) 1999 American Institute of Physics. [S0021-9606(99)30946-6].