where V and Q j are the potential energy of corresponding electronic excited states and the coordinate of the j-th normal mode, respectively. The VG approximation assumes that the excitedstate potential energy surface (PES) is just a shifted PES of the ground state, meaning that the electronic excitation does not significantly change the shape of the PES. For DPND molecules, the VG approximation should be suitable due to their planar and rigid structures according to our previous experience. In the absorption spectral calculation, we need to know the transition dipole operator, which is given by μ