Ab - initio non-adiabatic couplings among three lowest singlet states of H₃⁺: Construction of multisheeted diabatic potential energy surfaces

We calculate the adiabatic potential energy surfaces and non-adiabatic interactions among the three lowest singlet states (1¹ A' , 2¹ A' and 3¹ A' ) of H₃ ⁺ in hyperspherical coordinates for a fixed hyperradius, ρ = 9 bohr as functions of hyperangles, θ (0 < θ < 90°) and ϕ (0 < ϕ < 360°). All ab initio calculations are performed using MRCI level of methodology implemented in quantum chemistry package, MOLPRO. The ground (1¹ A' ) and the first excited (2¹ A' ) states exhibit several conical intersections as functions of ϕ for θ > 70°. Subsequently, we carry out adiabatic to diabatic transformation (ADT) to obtain ADT angles for constructing single-valued, continuous, smooth and symmetric 3 × 3 diabatic potential energy matrix to perform accurate scattering calculations. (paper)