A geometry-dependent Huckel Hamiltonian is defined for conjugated hydrocarbons. The model leads to st electron energies that can be coupled with the classical additive a potential. The use of analytical expressions for the first and second derivatives of the energy allows us to address the potential hypersurfaces and vibrational properties of large conjugated hydrocarbons with reduced computational effort. The method is tested on various conjugated systems ranging from linear to polycyclic hydrocarbons. The geometries and vibrational spectra are in good agreement with experimental data, which opens the field of interpretation and anticipation of vibrational spectroscopy of very large conjugated systems such as polycondensed aromatic hydrocarbons.