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Various levels of ab initio theory using various basis sets have been tested for the energy, structure, and harmonic vibrational frequencies of the water monomer. The level of the single, double, and perturbative triple excitation coupled-cluster method [CCSD(T)] using a large basis set (O:13s,8p,4d,2f/ H:8s,4p,2d) reproduced the experimental harmonic vibrational frequencies of the water monomer within the error of 0.6 cm-1. Other calculational methods seem to have inherent errors in predicting vibrational frequencies. Even with the above large basis set significant differences between symmetric and asymmetric stretching frequencies were found at various levels of calculational method including the commonly used Möller–Plesset 2nd (MP2) and 4th (MP4) order perturbation theories. The harmonic vibrational frequencies and force constants of the water dimer at various levels of ab initio theory have also been studied, and their shifts in the dimer relative to the monomer are discussed. At the Hartree–Fock (HF) level frequency shifts in the dimer relative to the monomer as well as hydrogen bonding strength in the dimer are underestimated, while at the MP2 level these are overestimated. The values at the CCSD(T) level seem to be reliable. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The global minimum energy structures of the water hexamer predicted by widely used analytic water potentials are very different from each other, while the cyclic hexamer does not appear to be a low-lying energy structure. However, high levels of ab initio calculation predict that a number of low-lying energy conformers including the cyclic conformer are almost all isoenergetic due to the balance of two-body and nonadditive interactions. For modeling of water potentials, we suggest that the binding energy of the dimer be between -5.0 and -4.7 kcal (mol dimer)-1, while the three-body corrections be taken into account to a large extent. [ABSTRACT FROM AUTHOR]

The correlation of various properties of water clusters (H[sub 2]O)[sub n=1-10] to the cluster size has been investigated using extensive ab initio calculations. Since the transition from two dimensional (2-D) (from the dimer to pentamer) to 3-D structures (for clusters larger than the hexamer) is reflected in the hexamer region, the hexamer can exist in a number of isoenergetic conformers. The wide-ranging zero-point vibrational effects of the water clusters having dangling H atoms on the conformational stability by the O-H flapping or proton tunneling through a small barrier (∼0.5 kcal/mol) between two different orientations of each dangling H atom are not large (∼0.1) kcal/mol). Large dipole moments (>2.5 D) are found in the dimer and decamer, and significant dipole moments (∼2 D) are observed in the monomer, hexamer, and nonamer. The polarization per unit monomer rapidly increases with an increasing size of the cluster. However, this increase tapers down beyond the tetramer. The O-H vibrational frequencies serve as sensitive indicators of the status of proton donation ("d") and acceptance ("a") (i.e., the structural signature of H-bond type) for each water monomer in the cluster. In general, the magnitudes of the O-H frequencies (ν) for each cluster can be arranged in the following order: ν[sub 3][sup da] (single donor-single acceptor) ≅ν[sub 3][sup daa] (single donor-double acceptor) >ν[sub 3][sup dda] (double donor-single acceptor) >ν[sub 1][sup dda]>ν[sub 1][sup da]> (or ≅) ν[sub 1][sup daa]. The increase in the cluster size has a pronounced effect on the decrease of the lower frequencies. However, there are small changes in the higher frequencies (ν[sub 3][sup da] and ν[sub 3][sup daa]). The intensities of ν[sub 1][sup daa] and ν[sub 1][sup da] are very high, since the increased atomic charges can be correlated to the enhanced H-bond relay effect. On the other hand, the intensities of the ν[... [ABSTRACT FROM AUTHOR]