Proton tunneling in malonaldehyde (MA) is investigated. Three-dimensional potential energy surfaces spanned by the O...H...O fragment of MA were generated based on DFT and reparameterized semiempirical calculations. The energy splitting of 42.3 (15.3) and 24.3 (5.3) cm(-1) at the DFT and semiempirical levels, respectively, find good agreement with the experimental values. The splitting is reduced to 36.4 (12.5) cm(-1) and to 18.6 (2.9) cm(-1) by introducing an effective mass for the heavy atoms, We show that the potential barrier of 4.0 kcal mol(-1) is consistent with the observed H (D) tunneling splitting of 21.6 (2.9) cm(-1). (C) 2002 Elsevier Science B.V. All rights reserved.