Dressed local mode Hamiltonians for CH stretch vibrations

A local mode CH stretch Hamiltonian is developed that provides the advantages of a localised representation of the molecular vibrations yet retains the accuracy of the normal mode representation. The increased accuracy over traditional local mode approaches is achieved by using Van Vleck perturbation theory. This theory incorporates the effects of couplings to other degrees of freedom by altering the molecular coupling constants. The resulting dressed local mode Hamiltonian and dipole operators have forms that are identical to those of the initial Hamiltonian and dipole. The procedure for generating the parameters of these Hamiltonians is completely general, but its implementation is illustrated for the specific examples of molecules containing -CH2CH2- linkages and molecules with aromatic CH stretches. The implementation takes as its starting point the output of electronic structure frequency calculations. The differences between local mode and dressed local mode operators are particularly pronounced for the calculation of transition intensities.