Minimun energy paths of key reactions in the ISM involving H₂ and an abundant neutral (left column) and cation element (right column). In this plot, only the reaction of O⁺ with H₂ is exoergic for all v. We define the reaction coordinate as , minimizing the energy in the remaining internal degrees of freedom but for a collinear H–H–X geometry (except for C⁺ and N⁺, in which the angle also varies to show the depth of the well). Each panel shows the energy associated with the lowest-energy vibrationally excited levels of H₂ (v) (marked in red). Panels with an asterisk indicate the dominant ionization state of the element in diffuse neutral clouds and at PDR edges.