Harmonic vibrational frequencies of the water monomer and dimer: Comparison of various levels of ab initio theory

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Abstract
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.