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A (1+1)-dimensional model of H+2is employed to study the validity of the fixed and the classically moving nuclei approximations commonly applied in theoretical description of ionization of diatomic molecules in short, intense laser fields. Up to a certain threshold intensity the ionization probability is found to be in very good agreement between all models, even though there are some differences in the final electronic probability density. Beyond the threshold intensity the classical and fixed nuclei approximations are seen to be completely inadequate to describe the ionization process. A simple scaling relation for the threshold intensity is derived and shown to be in excellent agreement with the numerical results.