  The expected distribution of Cepheids within the instability strip is
affected by several model inputs, reflecting upon the predicted
Period-Luminosity relation. On the basis of new and updated sets of
evolutionary and pulsational models, we quantitatively evaluated the effects on
the theoretical PL relation of current uncertainties on the chemical abundances
of Cepheids in the Large Magellanic Cloud and on several physical assumptions
adopted in the evolutionary models. We analysed how the different factors
influence the evolutionary and pulsational observables and the resulting PL
relation. As a result, we found that present uncertainties on the most relevant
H and He burning reaction rates do not influence in a relevant way the loop
extension in temperature. On the contrary, current uncertainties on the LMC
chemical composition significantly affect the loop extension and also reflect
in the morphology of the instability strip; however their influence on the
predicted pulsational parameters is negligible. We also discussed how
overshooting and mass loss influence the ML relation and the pulsational
parameters. In summary, the present uncertainties on the physical inputs
adopted in the evolutionary codes and in the LMC chemical composition are
negligible for the prediction of the main pulsational properties; the inclusion
of overshooting in the previous H burning phase and/or of mass loss is expected
to significantly change the resulting theoretical pulsational scenario for
Cepheids, as well as the calibration of their distance scale. These systematic
effects are expected to influence the theoretical Cepheid calibration of the
secondary distance indicators and in turn the resulting evaluation of the
Hubble constant.
