where n i = 0, 1, 2..., ∞ is the quantum number for the PFS. Compared to the vacuum's Fock state |n , this PFS depends on the diabatic state (or more generally, the eigenstate of μ) of the molecule, and the position of the nuclei (through the R dependence in µ i (R)). Due to the electronic statedependent nature of the polarization, the PFS associated with different electronic diabatic states become non-orthogonal, i.e., n i |m j = δ ij δ nm .