Born-Huang approximation wave function

In the two-electronic-state Born-Huang expansion, the full-Hilbert space of adiabatic electronic states is approximated by the lowest two states and furnishes for the corresponding electronic wave functions the approximate closure relation... 

Equation (6.130), which we will call the complete non-relativistic Hamiltonian, contains terms which couple the electronic and nuclear motions, making it impossible to obtain exact eigenfunctions and eigenvalues. This is where the Born-Oppenheimer approximation enters, in a method suggested by Bom and Huang [46]. We choose to expand the complete molecular wave function as the series... 

---