Classical dynamics of hydrogen bonded systems : water clusters

The minimum energy structures and the classical dynamics of (H2O)n clusters, with n=3–6,8, have been investigated with the potential function of Cieplak, Kollman, and Lybrand. The potential is the sum of a pairwise additive part plus a polarization term calculated iteratively from the permanent and induced dipole moments. It is found that at the minimum energy structures the polarization energy makes a contribution of about 13% in the total energy. Caloric curves and rms fluctuations of the oxygen–oxygen bond lengths show that the tetramer, with a tetragonal minimum geometry, and the octamer, with a cubic minimum geometry, are more stable clusters than the pentamer and hexamer. The geometries of absolute minima for the trimer, tetramer, and octamer agree with those predicted by other potentials. For the pentamer and hexamer the present potential predicts floppy type structures instead of cyclic ones, which are supported by other functions and ab initio calculations. The dynamics of water clusters is chaotic with a characteristic time for forgetting the initial conditions equal to the period of the stretching mode of the hydrogen bond. Phenomena of coexistence appear for the hexamer and larger clusters for which classes of minima of different geometrical type exist. For all studied clusters the power spectra of the oxygen velocity autocorrelation function are characterized by two frequency bands; one around 30 cm−1, torsional, and one at 200 cm−1, stretching. Generally, no significant influence on the dynamics of the clusters is found by the inclusion of polarization interaction, although for the hexamer and octamer polarization energy results in interchanging the geometries of absolute minima.