Ab initio calculations that include correlation energy have been performed to study the first two vertical ionic states of H2S—H2S, PH3—H2S, and FH—H2S hydrogen bonded molecules and the subsequent rearrangement processes. In all cases, the first two ionic states are the 2A’ and 2A” states depending on whether the ionization is located in the proton acceptor or in the proton donor, respectively. If ionization is produced in the proton donor monomer, the dimer evolves to a proton transfer complex, while if ionization is produced in the proton acceptor the dimer dissociates and then evolves to a three-electron hemibond complex. This hemibond structure is found to be the most stable isomer for all the systems. The obtained results are compared with those obtained from comparable calculations performed previously for the first-row H2O—H2O, NH3—H2O, and FH—H2O hydrogen bonded molecules. © 1995, American Chemical Society. All rights reserved.