Semiempirical AM1 and MNDO-PM3 as well as MPn/4-31G (n=2-4) calculations were done on the hydrogen-bonded intermediate complexes and the transition state for the unassisted addition of one water molecule to formaldehyde in order to assess the similarity of the three models in the study of chemical reactions. The errors inherent in the low-level ab initio calculation were estimated both by extension of the basis set and by correction of the basis-set-superposition error (BSSE). Basis-set extension was performed by using MPn/6-31G(d,p) calculations. BSSE was corrected both at the Hartree-Fock and the post-Hartree-Fock level using the counterpoise method and accounting for deformation of the fragments. Drawbacks of the semiempirical methods are reported and analysed in relation to the ab initio results. © 1990.