An Improved Semiempirical MO PM3 Method for Hydrogen‑Bonded Systems
Jožica Dolenc and Jože Koller*
Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, SI-1000 Ljubljana, Slovenia
Semiempirical methods are known to show artifacts in predicting the interaction energies and structures of hydrogen-bonded systems. The origin of the artifacts is the correction function of the core-core interaction term, which was introduced to adjust the repulsion between the nuclei. Recently a new PM3 core-core correction function based on the atom-atom parameters has been proposed and the parameters for O‑O, O‑H, and H‑H interactions have been reported. In this work the new core‑core correction function has been implemented in the semiempirical molecular orbital package MOPAC97. The test calculations have been performed on the following simple systems: a protonated water dimer, a cyclic water trimer, a cyclic water tetramer, four conformers of a water dimer, a water monomer, an oxonium ion, hydrogen peroxide, and hydrogen trioxide. The results were compared to the values obtained with the standard PM3 method, ab initio methods, and to experimental data, where available. In all of the examples, except for the case of hydrogen peroxide, the modified semiempirical PM3 method provides a significant improvement of the description of the geometries and interaction energies of the test systems, indicating that the reparametrization of the core‑core interaction term may play an important role in the future development of semiempirical methods.
Key words: core-core repulsion, hydrogen bonds, semiempirical methods, PM3, MOPAC, water