Ab initio and semiempirical molecular orbital calculations on aggregates containing up to 36 molecules of acetic acid are presented. Aggregation is considered in three directions : (a) H-bonding interactions involving O-H...O add C-H...O interactions to form "chains"; (b) "stacking" of the chains through weak C-H...O interactions; and (c) assembly of the "stacks" via other C-H...O H-bonding interactions to form "microcrystals". The results provide the first evidence of cooperativity for interactions between different directions as well as within each of the three directions. These cooperative effects are manifest both in the energies of stabilization and in the inter- and intramolecular geometric parameters. An analysis of the interactions between chains (to form stacks) and between stacks (to form microcrystals) shows that pairwise interactions (between either chains or stacks) significantly underestimate the respective interaction energies. The calculations agree reasonably well with the experimental crystal structure but only after all three directions are considered. The results are discussed in terms of the intermolecular forces that operate in processes of molecular recognition and self-assembly such as crystal formation. It is suggested that nonadditive cooperative effects can be extremely important to successful modeling of aggregation and molecular recognition.