Accidental conical intersections in H 2 Cl + ( 1 A ′)

Conical intersections in C s , C 2 v, and linear configurations for the 1 A′ states of the H 2 Cl + molecule are investigated with the help of a 10 × 10 model Hamiltonian matrix in a valence-bond basis. The line of intersection (linea) between states 2 and 3 is traced from one end point, an intersection between two states of 1 B 2 symmetry in a C 2V configuration, to the other end point, an intersection between two 1 Σ + states in a linear H—H—Cl configuration. The linea consists of 5 branches having symmetries C s , C s , C ∞v , C 2v , and C s . A linea between states 3 and 4 starting at a crossing of 1 Σ + and 1 Π states in a linear C ∞v configuration consists of three branches (C s , C 2V , C s ) leading to the asymptotic region Cl + H 2 . The starting point of linea 3/4 is connected to the asymptotic region H + HCl via a linea in the linear configuration. Lineae of C s symmetry could be found by examining the eigenvalues of the Hessian matrix of second derivatives along a line of intersection in a higher symmetry (C 2v or linear), a useful method for locating accidental intersections that lends itself readily to ab initio calculations. The existence of the lineae could be related to physical properties of the molecule, such as the dipole moment, indicating that the model Hamiltonian does reflect the true physical behaviour.