ABINITIO STUDIES OF THE WATER DIMER USING LARGE BASIS-SETS - THE STRUCTURE AND THERMODYNAMIC ENERGIES

Ab initio calculations with various large basis sets have been performed on the water dimer in order to study the structure, energetics, spectra, and electrical properties. As a reference system, the calculations of the water monomer were also performed. The second order Moller-Plesset perturbation theory (MP2) using a large basis set (O:13s,8p,4d,2f/H:8s,4p,2d) well reproduces various water monomer experimental data except for the somewhat underestimated absolute energy and hyperpolarizability. The monomer energy calculated with the fourth-order Moller-Plesset perturbation theory (MP4) with the above basis set is -76.407 hartrees, which is only 0.073 hartree above the experimental energy. To compare the theoretical and experimental dimer structures and thermal energies accurately, we summarized the quantum statistical thermodynamic quantities with corrections for anharmonic vibration, rotation, rotation-vibration coupling, and internal rotation. With the correction for the anharmonic binding potential and rotation, the predicted interoxygen distance of the dimer is 2.958 angstrom, which is so far the closest to the experimental value approximately 2.976 angstrom. The predicted dimer dipole moment is 2.612 D, which is the first agreement with experiment (2.60-2.64 D). The predicted frequency shift of the dimer with respect to the monomer is in good agreement with experiment. With the MP2 calculation using the large basis set, the basis set superposition error correction (BSSEC) of the dimer is only 0.33 kcal/mol, which is by far the smallest among the MP2 results reported. Without BSSEC, the predicted binding energy, enthalpy, free energy, and entropy are all in good agreement with experiment within the error bounds, whereas with BSSEC, some of them seem to be slightly off the experimental error bounds. Nevertheless, the results with BSSEC can be more reliable than those without BSSEC.