Ab initio molecular orbital calculations were performed on the C-H ... O hydrogen bonded dimer pattern of the fragment O=C-C(=C)-H as occurring in 1,4-benzoquinone. The computations were combined with a statistical analysis of published crystal structures containing this dimer. For computational reasons, the dimer was approximated by dimers 1,4-benzoquinone-propenal and propenal-propenal. The optimal geometry obtained at the SCF+MP2 level of theory is very close to the mean geometry observed in crystals. The total binding energy Delta E at the optimal geometry is calculated as -17.9 kJ/mol. Each C-H ... O hydrogen bond contributes about -6 kJ/mol, and the rest comes from stabilizing electrostatic interactions between the carbonyl groups. The potential energy surface has a broad shape at the minimum, allowing considerable geometric variations with only slight energetic disadvantages. The dimer geometries observed in crystals (n = 53) are all in the low-energy region of the potential energy surface. Only 4 of the 53 dimers have calculated Delta E values more than 3 kJ/mol above the global minimum. The dimer with the least favorable geometry has a Delta E value about 7.0 kJ/mol above the global minimum.