A variational method for the calculation of spin-rovibronic energy levels of triatomic molecules with three interacting electronic states

A general method is presented for the evaluation of the spin-rovibronic ene rgy levels of triatomic molecules with up to three interacting potential en ergy surfaces. The full theory is outlined in detail, both for singlet and for doublet electronic states and the method is then applied to the benchma rk example of C2H. High quality multireference configuration interaction ca lculations have been carried out to generate the 3-dimensional near-equilib rium adiabatic potential energy surfaces of the three lowest 1(2)A '(X(2)Si gma+), 2(2)A '(A(2)Pi), 1(2)A "(A(2)Pi) electronic states of C2H, and the p air of interacting states of A ' symmetry have then been diabatized. Result s are presented for J up to 7/2 and compared with gas-phase high resolution experimental results for energies up to 5600 cm(1).