The proton tunneling reaction in malonaldehyde at low temperatures is investigated. The principal aim of this study is to find the optimal tunneling path at 0 K in the framework of the semiclassical theory with a global optimization method. An amount of 11366 ab inito points was determined in the reaction swath (i.e., the conformational space enclosed by the minima and the transition state) of malonaldehyde. With a simulated annealing approach, the path with the smallest integral of the imaginary action through the swath from minimum to minimum was determined. Surprisingly the optimal tunneling path was found to be quite far off the large curvature tunneling path [i.e., the straight connection of the two minima large-current tunneling (LCT path)]. At the beginning, it is following the minimum energy path (MEP) (i.e. the path with the lowest energy connecting the two minima and passing through the transition state), and then it is describing a curved path through the reaction swath. This curve was determined several times with different annealing schemes, which ended up with the same result-the tunneling path is proceeding close to the MEP rather than to the LCT path. Along the optimal tunneling path, the ground-state tunneling splitting was calculated with a new semiclassical method introduced in an accompanying study [C. S. Tautermann, A. F. Voegele, T. Loerting, and K. R. Liedl, J. Chem. Phys. 117, 1967 (2002), following paper]. Another focus of investigation was the influence of deformation of the tunneling paths and a general scheme of determining an approximated optimal tunneling path at 0 K is introduced.