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Thermodynamic and structural properties of a coarse-grained model of methanol are examined by Monte Carlo simulations and reference interaction site model (RISM) integral equation theory. Methanol particles are described as dimers formed from an apolar Lennard-Jones sphere, mimicking the methyl group, and a sphere with a core-softened potential as the hydroxyl group. Different closure approximations of the RISM theory are compared and discussed. The liquid structure of methanol is investigated by calculating site-site radial distribution functions and static structure factors for a wide range of temperatures and densities. Results obtained show a good agreement between RISM and Monte Carlo simulations. The phase behavior of methanol is investigated by employing different thermodynamic routes for the calculation of the RISM free energy, drawing gas-liquid coexistence curves that match the simulation data. Preliminary indications for a putative second critical point between two different liquid phases of methanol are also discussed.

Using Monte Carlo computer simulations, we show that a simple isotropic water-like model with two characteristic lengths can reproduce the hydrophobic effect and the solvation properties of small and large non-polar solutes. Influence of temperature, pressure, and solute size on the thermodynamic properties of apolar solute solvation in a water model was systematically studied, showing two different solvation regimes. Small particles can fit into the cavities around the solvent particles, inducing additional order in the system and lowering the overall entropy. Large particles force the solvent to disrupt their network, increasing the entropy of the system. At low temperatures, the ordering effect of small solutes is very pronounced. Above the cross-over temperature, which strongly depends on the solute size, the entropy change becomes strictly positive. Pressure dependence was also investigated, showing a "cross-over pressure" where the entropy and enthalpy of solvation are the lowest. These results suggest two fundamentally different solvation mechanisms, as observed experimentally in water and computationally in various water-like models.

An interaction model with core-softened potential in three dimensions was studied by Monte Carlo computer simulations and integral equation theory. We investigated the possibility that a fluid with a core-softened potential can reproduce anomalies found experimentally in liquid water, such as the density anomaly, the minimum in the isothermal compressibility as a function of temperature, and others. Critical points of the fluid were also determined. We provided additional arguments that the old notion, postulating that only angular-dependent interactions result in density anomaly, is incorrect. We showed that potential with two characteristic distances is sufficient for the system to exhibit water-like behavior and anomalies, including the famous density maximum. We also found that this model can properly describe the hydrophobic effect.

Integral equation approaches, based on the Ornstein-Zernike equation, provide a fast way to calculate phase diagrams and thermodynamic properties of systems as opposed to time-consuming and computationally expensive computer simulations. However, when employing integral equations it is necessary to introduce simplifications. The Ornstein-Zernike equation merely relates two unknown functions h(r) and c(r), and another relation (closer) between these two functions is needed. The later function cannot be obtained in a closed form and it is always in some approximations. Various approximations exist with each of its own advantages and disadvantages. In this work we extensively tested hyper-netted chain, Percus-Yevick, Kovalenko-Hirata, and Rogers-Young closure on an interaction model with core-softened potential. Convergence domain was established for each method. We calculated pair distribution functions, pressure, and excess energy. Results were compared with Monte Carlo simulation results and literature data from molecular dynamics simulations.

In-depth knowledge of water-water potential is important for devising and evaluating simple water models if they are to accurately describe water properties and reflect various solvation phenomena. Water-water potential depends upon inter-molecular distance, relative orientation of water molecules, and also local environment. When placed at a favorable distance in a favorable orientation, water molecules exhibit a particularly strong attractive interaction called hydrogen bond. Although hydrogen bond is very important for its effects on the elements of life, industrial applications, and bulk water properties, there is no scientific consensus on its true nature and origin. Using quantum-mechanical methods, hydrogen bond strength was calculated in different local environments. A simple empirical linear relationship was discovered between maximum hydrogen bond strength and the number of water molecules in the local environment. The local environment effect was shown to be considerable even on the second coordination shell. Additionally, a negative linear correlation was found between maximum hydrogen bond strength and the distance, at which it was observed. These results provide novel insights into the nature of hydrogen bonding.