Cassandra is an open source atomistic Monte Carlo software package that is effective in simulating the thermodynamic properties of fluids and solids. The different features and algorithms used in Cassandra are described, along with implementation details and theoretical underpinnings to various methods used. Benchmark and example calculations are shown, and information on how users can obtain the package and contribute to it are provided. © 2017 Wiley Periodicals, Inc.

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The wetting behavior of an ethanol−water droplet is investigated on graphitic smooth and rough surfaces using molecular dynamics simulations. On a smooth surface, ethanol molecules prefer to stay at the vapor−liquid and solid− liquid interfaces. The contact angle of a droplet on a smooth surface decreases with an increase in the ethanol concentration from 0 to 30 wt %. The corresponding line tension increases from 3 × 10 −11 to 9.4 × 10 −11 N at 300 K. The critical weight percentage for complete wetting is found to be approximately 50%. In the case of a textured graphite surface, with the addition of ethanol molecules, the Cassie− Baxter state of a drop is transformed into the Wenzel state via the partial Wenzel state, with ethanol molecules filling the rough region, leading to an increase in its wettability. A linear relation of 1 + cos θ with the roughness parameter associated with the Cassie−Baxter and Wenzel states is observed, indicating that the solid− liquid interfacial tension is directly proportional to the roughness parameter. This behavior is akin to that seen for the case of pure liquid. The hydrogen bonding and density profile are analyzed to understand the wetting states of the blended drop.

Prewetting transitions are studied for Lennard-Jones (LJ) based dimer forming associating fluids, on a structureless surface represented by LJ 9-3 type potential, for various association strengths using grand-canonical transition matrix Monte Carlo (GC-TMMC) and histogram reweighting techniques. Occurrences of prewetting transition are observed for association strengths: epsilon(af)=2.0, 4.0, 6.0, 8.0, and 10.0. Structural properties, monomer fraction, and orientation order profile of thin-thick film of one-site associating fluids are presented. Wetting temperature, T(w), and prewetting critical temperature, T(pwc), increases with increasing association strength, which is in agreement with the results of the density functional theory (DFT). Length of prewetting line, on the other hand, is found to decrease first with increasing association energy until epsilon(af)=8.0 and subsequently found to increase substantially for epsilon(af)=10. This behavior is contrary to the prediction from the DFT. We observe that the boundary tension of thin-thick film via GC-TMMC and finite size scaling exhibits a maximum with respect to association strength.