Structural changes in the water tetramer. A combined Monte Carlo and DFT study.

The heat capacity curve of the water tetramer has been calculated at various levels of theory and over a broad range of temperatures, T = 50-200 K. Parallel-tempering and multiple-histogram Monte Carlo methods have been used and combined with the Density Functional Theory calculations of intra-cluster interactions via the Boltzmann-reweighting approach. It is demonstrated that such a combination can yield well converged thermodynamic data even for a modest number of sample configurations, which makes the methodology particularly appropriate for the inclusion of quantum chemistry calculations in Monte Carlo simulations. The B97-1 exchange-correlation functional has been used in the present work together with augmented correlation-consistent basis sets of atomic orbitals up to triple-zeta quality. A structural change has been detected in the water tetramer in the temperature range considered. It is demonstrated that the change corresponds to the onset of rotations of the water molecules around hydrogen bonds with an almost rigid skeleton of the cluster (square cycle) conserved.