A second strategy was proposed by Hanson-Heine et al. which is based on quantumchemical calculations. From a harmonic frequency calculation followed by a localization of the relevant normal modes (see Section 2.1), the localized-mode frequencies ωi and localmode transition dipoles μi can be extracted directly. The harmonic coupling constants J ij can be obtained as elements of the coupling matrix J = ( Hsub ) 1/2 , i.e., the matrix square root of the Hessian with respect to localized modes. These parameters are then used in combination with an empirical anharmonic shift ∆ to construct the vibrational exciton Hamiltonian. We will refer to this parametrization as "∆-exciton model".