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Vibrational force fields for the ground states of 1,3-butadiene, 1,3,5-hexatriene, and 1,3,5,7-octatetraene are calculated from extended Pariser–Parr–Pople CI theory. For the ground state of butadiene, the principal π-system force constants are compared to those determined by empirical, other semiempirical, and ab initio methods. The calculated vibrational frequency shifts for a series of deuterium substituted butadienes are found to be in good agreement with experiment. Trends in the principal π-system force constants of the polyenes for the ground states of ethylene through octatetraene are described, including the effects of introducing cis linkages in the polyene chain and of using single- vs single-plus-double-excitation CI. Correlations between the force constants and the calculated vibrational frequencies are discussed, and current assignments of the vibrational spectra of the polyenes are examined in view of the theoretical results. [ABSTRACT FROM AUTHOR]