Charge exchange and chemical reaction in the H2 ++H2 system. I. Characterization of the potential energy surfaces and nonadiabatic regions

Potential energy surfaces for the H₄ ⁺ system are calculated by the valence bond diatomics-in-molecules method in the zero overlap-of-atomic-orbitals approximation. The adiabatic potential energy surfaces are obtained by the diagonalization of an 8×8 Hamiltonian matrix and are ideally suited for classical trajectory studies involving electronic transitions. The ground state surface of H₄ ⁺ is discussed and particular emphasis is given to those regions of configuration space for which this surface avoids an intersection with that of the first excited electronic state.