The measured absorption and emission spectral lineshapes and linewidths are reproduced by augmenting this model with an effective low-frequency vibration ( ) , mostly describing the conformational degrees of freedom along the polymer chains and including residual static disorder ( ), see below. and are the exciton-phonon coupling constants corresponding to the high-and low-frequency modes, and | ⟩ represents a local diabatic basis state of the stack where chain is electronically excited while all other chains remain in their ground states. Meanwhile, the nuclear dynamics is determined by a classical Hamiltonian