Ab initio calculations of coupled potential energy surfaces for the Cl(2P3/2,2P1/2)+ H2 reaction

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Three-dimensional potential energy surfaces for the entrance channel of the title reaction have been computed using accurate multi-reference configuration wavefunctions and a very large basis set. The three adiabatic potential surfaces correlating asymptotically with Cl(2P) + H2 (2Σ+ and 2Π for collinear geometries) have been transformed to a diabatic representation, which leads to a fourth coupling potential for non-linear geometries. In addition, the spin–orbit coupling surfaces have been computed using the Breit–Pauli Hamiltonian. The six resulting potentials (four electrostatic and two spin–orbit) have been fitted to analytical functions. The diabatic 2Π potential has a minimum in the entrance channel at collinear geometries, while the diabatic 2Σ potential has a well for a T-shaped (C2v) structure. This different anisotropy leads to a conical intersection of the two 2A′ (2Σ+, 2Π) adiabatic potentials at intermediate H2–Cl distances. The adiabatic spin–orbit potentials, which correlate asymptotically with Cl(2P1/2,3/2) + H2 are obtained by diagonalizing the full electrostatic + spin–orbit Hamiltonian. Spin–orbit coupling increases the adiabatic barrier height by 0.84 kcal mol−1. Implications for the non-adiabatic reaction dynamics are discussed.