Applications of quantum chemistry approaches are considered to elucidate the mechanisms of structural phase transitions in H-bonded molecular crystals. The KH2PO4 (KDP) family and squaric acid (H(2)SQ) are studied as examples. Proton rearrangements within H-bonds O-H ... O reversible arrow O ... H-O are treated in terms of variation of the oxygen atom states O-O-H reversible arrow O-O ... H on the basis of the vibronic theory of ligand substitution effects in molecules and complexes. Proton positions within H-bonds are described using the pseudospin formalism. For the energy of crystal this approach results in the Ising model, where the Ising and Slater parameters are expressed in terms of the MO structure of molecular units of crystal. Quantum-chemical calculations are used to determine the MO structure of these units with dependence on the proton positions within the H-bonds. The values of the Slater parameters derived are in reasonable agreement with the experimental data, and the compositional trends for T-c behavior observed for the KDP-type compounds are explained. Applications of the direct quantum-chemical modeling to study of properties of H-bonded crystal are briefly described.