An ab initio investigation of high-level accuracy on the magnitude of barriers to linearity, inversion, and internal rotation in the potential energy surfaces of water, ammonia, and ethane has been carried out using various bases and correlation treatments. It is shown that the increase of basis set size generally reduces the calculated barrier heights with correlation methods, and the proper account of one-particle basis set is necessary for the observation of systematic change of the calculated barrier heights according to electron correlation. A careful examination of core correlation effect on the calculated barrier height shows that freezing the core orbitals in correlated calculations would yield a slightly higher value than the actual barrier height, and it is important to include appropriate core-correlating functions in the basis for accurate prediction of relative energies such as internal barriers of polyatomic molecules. From this study, we predict that the barrier height to internal rotation in ethane would not exceed 960 cm(-1), which appears to be quite lower than previous theoretical results reported so far. The barrier height to linearity in water was found to be least convergent with basis set, probably due to the presence of the most lone pair electrons in the molecule.