Photoabsorption spectra of small HgN clusters (N = 2–5) have been calculated using a diatomics-in-molecules interaction model and an atoms-in-molecules approach for transition probability calculations.

Classical parallel-tempering Monte Carlo simulations in the isothermal-isobaric ensemble were carried out for the (H2O)20 and Ar(H2O)20 clusters, over a wide range of temperatures (30-1000 K) and pressures (3 kPa-10 GPa) in order to study their thermodynamic properties and structural changes. The TIP4P/ice water model is employed for the water-water interactions, while both semiempirical and ab initio-based potentials are used to model the interaction between the rare-gas atoms and the water molecules. Temperature-pressure phase diagrams for these cluster systems were constructed by employing a two-dimensional multiple-histogram method. Structural changes were detected by analyzing the heat capacity landscape and the Pearson correlation coefficient profile for the interaction energy and volume. Those at high pressure correspond to solid-to-solid transitions and are found to be related to clathrate-like cages around the Ar atom. It is also shown that the formation and thermodynamic stability of such structures are determined by the intermolecular interaction between the rare-gas atoms and the host water molecules.

Classical thermodynamic properties of a size-specific Kr clathrate-like cluster, modeled by an ab initio and a semiempirical Kr-water potential, were computed from parallel-tempering Monte Carlo simulations. The temperature and pressure dependence of the cluster's heat capacity was studied, and phase diagrams were constructed using a multiple-histogram method. By associating the heat capacity (maxima) and the Pearson correlation coefficient (minima/maxima) values with the phase transitions, attempts were made to identify such changes to particular cluster structures. Various isomers were computed by local optimizations of the interaction enthalpy at a set of randomly selected configurations at each temperature-pressure grid point. Their energy distributions and relative abundances were then employed to assign the observed phase transitions of the cluster. It was found that the structural changes at high pressures are related to Kr clathrate-like cages, such as those of the sI, sII and sH hydrates, as well as a new one at higher pressures, formed by tetragons and pentagons. Such transitions, at low temperatures and as pressure increases, are related to the topology of the intermolecular interactions, that are getting accessible by the sampling in the MC simulations, through the employed volume model.

Thermal properties and structures of the water cluster containing fifteen molecules, either pure or doped with methane, are studied via classical parallel tempering Monte Carlo calculations in the isothermal-isobaric ensemble. The main emphasis is on structural transformations the cluster undergoes with increasing temperature and pressure. A simple TIP4P interaction model is employed for water and the unified-atom approximation with a Lennard-Jones potential is used to model the methane-water interaction. The results are compared with the data obtained recently for zero temperature via evolutionary algorithm calculations [Hartke, J. Chem. Phys., 2009, 130 art. no. 024905].