corresponding to the set of electronic coordinates q = {q i }, the set of nuclear coordinates Q = {Q k } with nuclear masses {M k }. In some treatments, one may also incorporate the spin-orbit interaction into the Hamiltonian, however for most systems without heavy atoms, first-order spin-orbit coupling is weak, and can therefore be ignored. In terms of the partial Schrödinger equation, the exact wavefunction ψ (q, Q) can be expressed as :