The reduced mass as a function of the ring-puckering coordinate for cyclobutane, 1,3-disilacyclobutane, cyclobutyl fluoride, cyclobutyl chloride, and acetylene is calculated by a relaxed model. For this model, the Cartesian coordinates, and their derivatives with respect to the puckering coordinates, are obtained based on ab initio geometry optimization calculations at several puckering angles. The reduced mass function calculated by this model is very different from those obtained previously. The effect of the difference between this relaxed model and the semirigid model on the potential function is demonstrated and discussed.