To elucidate the stable structures and electronic properties of hydrogen-bonded 4-hydroxyazobenzene (HOAB) dimers, we performed ab initio molecular orbital calculations based on the density functional theory. The obtained stable structures and total energies for the 15 types of dimers indicate that the trans-trans HOAB dimers are at least 10 kcal/mol more stable than the other cis-cis, cis-trans and trans-cis dimers. In addition, it was found that the dimer of tautomerized trans-HOAB has similar stability to the trans-trans HOAB dimers, suggesting that the tautomerized trans-HOAB dimer contributes to the unique photochemical and spectroscopic behavior of HOAB. (c) 2005 Elsevier B.V. All rights reserved.