A double many-body expansion of the two lowest-energy potential surfaces and nonadiabatic coupling for H₃

We present a consistent analytic representation of the two lowest potential energy surfaces for H₃ and their nonadiabatic coupling. The surfaces are fits to ab initio calculations published previously by Liu and Siegbahn and also to new ab initio calculations reported here. The analytic representations are especially designed to be valid in the vicinity of the conical intersection of the two lowest surfaces, at geometries important for the H+H₂ reaction, and in the van der Waals regions.