Sum and density of states for anharmonic polyatomic molecules. Effect of bend–stretch coupling

A model anharmonic molecular Hamiltonian is used to study the effect of nonseparable bend–stretch coupling on sum and density of states. A bond-energy–bond-order (BEBO) type method is used to couple bending and stretching potential energy terms of a local mode Hamiltonian. The model Hamiltonian is applied to the hydrocarbon fragments H–C≡C, H–C=C, and H–C–C. The results show significant deviation from the independent oscillator model, especially when the stretches are taken to be the Morse type and the energy is close to the dissociation limit.