Structures and energetics of the water heptamer: Comparison with the waterhexamer and octamer

In spite of a spate of studies of various water clusters, a few theoretical studies on the water heptamer are available. State-of-the-art ab initio ca lculations are thus carried out on twelve possible water heptamer structure s to explore the conformation as well as spectroscopic properties of this w ater cluster. Two three-dimensional cagelike structures comprised of seven- membered cyclic rings with three additional hydrogen bondings were found to be the lowest-lying energy heptamer conformers. The global minimum energy structure was found to be 0.5 kcal/mol lower than the other. The zero-point energy uncorrected and corrected binding energies of the global minimum en ergy structure are 55.2 and 37.9 kcal/mol, respectively. An almost two-dime nsional ring conformer, which is only 1 kcal/mol above the global minimum a t 0 K, could be more stable above 150 K. The vibrational spectra of differe nt heptamer conformers are discussed and compared with the spectra of the h examer and octamer water clusters. (C) 1999 American Institute of Physics. [S0021-9606(99)30118-5].