Structure and vibrational spectra of small water clusters from first principles simulations.

The structure and vibrational spectra of (H(2)O)(n) (n=2-5) clusters have been studied based on first-principles molecular dynamics simulations. Trends of the cluster structures with the cluster size show that water molecules in cluster are bound more tightly. The vibrational spectra as a function of cluster size and temperature are obtained using Fourier transformation of the velocity autocorrelation function. Results of the clusters in ground state show that when the cluster size increases, the librational peaks shift to blue and the bonded intramolecular OH stretching bands shift to red due to the clusterization and hydrogen-bond strengthening. Meanwhile, there are no significant shifts in the intramolecular bending and free OH stretching modes, indicating that the free hydrogen atoms are insensitive to the local bonding environment. The temperature-dependent vibrational spectra, which exhibit similar behaviors from the dimer to pentamer, show that there are significant broadenings of the spectra with temperature caused by thermal motions. Moreover, different bands shift to different directions, where librational bands shift to red while bonded OH stretching bands shift to blue, although the blueshifts are quite small for the dimer and trimer.