A systematic ab initio investigation has been carried out to examine the effects of a basis set and correlation method on barrier height io linearity of bent triatomic CH2 and H2O in their ground electronic states. The theoretical models employed varied from Hartree-Fock with minimal STO-3G to highly correlated quadratic configuration interaction method QCISD(T) with Dunning’s correlation-consistent aug-cc-pVQZ basis set. It is shown that the barrier height to linearity is very sensitive to the choice of the model and it is necessary to employ a proper theoretical model with large enough basis set to be assured of the convergence of the calculated barrier height. We found that calculated barrier height to linearity tends to be higher than experimentally determined barrier height in most cases, which is caused by the slower convergence of energy at linear saddle point geometry than at bent equilibrium geometry. The effect of frozen core approximation in correlated calculations is shown to always increase the barrier height to linearity slightly (less than 3% of total amount) from the value with the full activation of orbitals, regardless of the basis set or correlation method used in the study.