In this work, the vibrational force fields of hydrogen-capped oligoynes of increasing chain lengths are investigated by means of density functional theory calculations. It is shown that the interaction force constants between CC stretching coordinates decrease slowly with the distance between the two bonds considered. The consequence for the frequency dispersion of longitudinal optical (LO) phonons of an infinite polyyne chain is discussed and related to the observed behavior of the spectra of finite-size molecules. Effects of the exchange-correlation functional and of the basis set on the vibrational force constants are also investigated and the need for a scaling procedure is pointed out. Accordingly, new force fields which allow predictions in very good quantitative agreement with the available experimental data for oligoynes have been obtained, providing a sound assignment of alpha and beta lines.