One H-bonded (planar) and one stacked (vertical) structures of the formic acid tetramer were investigated using extended basis sets at the HF, B3LYP, PW91PW91, and MP2 levels of theory. At the HF, B3LYP, and PW91PW91 theoretical levels, the H-bonded structure is more stable than the stacked one. At the ab initio MP2 correlated level, the stacked structure is more stable than the H-Bonded one. (C) 2004 Elsevier B.V. All rights reserved.