Clearly, using the states | 1 ⟩ and | 2 ⟩ as basis will introduce off-diagonal elements in the Hamiltonian. Here, because of the relative simplicity of the H 2 + ion, the matrix elements can actually be calculated. The electronic Hamiltonian of H 2 + commutes with the point group inversion symmetry operation i. Using its symmetry properties, we can relate the diagonal and off-diagonal elements of the Hamiltonian as: