Vibrational softening of diatomic molecules
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An analysis of a model molecular oscillator is presented: a vibrating diatomic molecule carrying N0 electrons. The energy derivatives over the number of electron (N) and the deformation (Q), ∂ n /∂N n and ∂ n /∂Q n have been analyzed up to second order (n=2), including the appropriate mixed derivatives. The effect of coupling between distortion of the electron density induced by ΔN and the vibrational deformation of the molecule has been studied. Anharmonicity of the oscillator has been shown to be a possible result of that coupling; new relations between the parameters characterizing the anharmonicity of the oscillator and the energy derivatives at density functional theory level have been obtained. Ab initio calculations for a set of diatomic molecules have been performed, yielding values for all the derivatives discussed and demonstrating the effect of coupling with vibrations.
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