Modeling cusps in adiabatic potential energy surfaces using a generalized Jahn-Teller coordinate

Highlights: • A generalization of the Jahn-Teller coordinate proposed for modeling cusps on single-sheeted adiabatic PESs for triatomics is extended to any crossing seam. • The model is applied to the ${}^2{\text{A}}^{″}$ state of NO₂ and ${\text{HN}}_2\left({}^2{\text{A}}^{\prime }\right)$. • It can be of great utility for reaction dynamics, since fits with smooth functions are problematic in describing conical intersections. A recent generalization of the Jahn-Teller coordinate proposed for modeling cusps on single-sheeted adiabatic potential energy surfaces for triatomics is extended to any type of crossing seam. The model is applied to the ${}^2{A}^{″}$ state of ${\text{NO}}_2$ and the more complicated ${\text{HN}}_2\left({}^2{A}^{\prime }\right)$ system, which shows permutationally equivalent and strongly curved seams.