Suehiro Iwata

Comparison of the sum of the characteristic factors for some of the typical hydrogen donor and acceptor pairs with the CT term/kJ mol ⁻¹ (the upper value) and the O⋯O distance/in cubic (H ₂ O) ₈ .

The hydrogen bonds and their networks in the water clusters (H₂O)₂₀ and (H₂O)₂₅ are characterized using the charge-transfer and dispersion terms for every pair of water molecules (W_a, W_d) in the clusters. The terms are evaluated by the perturbation theory based on the ab initio locally projected molecular orbitals (LPMO PT) developed by the present author. The relative binding energies among the isomers evaluated by the LPMO PT agree with those of the high level ab initio wave function based theories. A strong correlation between and for the hydrogen bonded pairs is found. The pair-wise interaction energies are characterized by the types of hydrogen-donor (W_d) and hydrogen-acceptor (W_a) water molecules. The strongest pair is that of the D2A1 water molecule as a hydrogen-acceptor and the D1A2 water molecule as a hydrogen-donor, where the DnAm water molecule implies that the water molecule has n hydrogen bonding O–H and m accepting H⋯O. The intra-molecular deformation as well as the O⋯O distance is also dependent on the types of hydrogen bonded pairs. The ring structures in the cluster are classified by the pattern of alignment of the hydrogen bonds. The lengthening of the hydrogen-bonding OH of W_d is strongly correlated with the charge-transfer energy.

The perturbation expansion based on the locally-projected molecular orbital (LPMO PT) was applied to the study of the hydrogen-bonded networks of water clusters with up to 16 molecules. Utilizing the local nature of the occupied and excited MOs on each monomer, the charge-transfer and dispersion terms are evaluated for every pair of molecules. The two terms are strongly correlated with each other for the hydrogen-bonded pairs. The strength of the hydrogen bonds in the clusters is further classified by the types of the hydrogen donor and acceptor water molecules. The relative energies evaluated with the LPMO PT among the isomers of (H₂O)₆, (H₂O)₁₁, and (H₂O)₁₆ agree very well with those obtained from CCSD(T) calculations with large basis sets. The binding energy of the LPMO PT is approximately free of the basis set superposition errors caused both by the orbital basis inconsistency and by the configuration basis inconsistency.

A multicanonical Monte Carlo simulation has been performed to investigate average structural changes in the water dimer with temperature. Potential energy was calculated by the ab initio MO method at the RHF/6-31G∗ level at each step of the simulation. By combining the reweighting technique with the multicanonical algorithm various thermodynamic properties have been evaluated over a wide range of temperature from a single production run. The technique is an ideal tool for studying temperature dependence properties, and the temperature dependence of the distribution functions of various geometric parameters has been examined in detail. It was found that the probability of finding structures similar to that of the transition state increases as the temperature is increased.