From code to thermodynamics: a molecular simulation perspective

Thermodynamic properties of matter have been studied by means of computer simulations for more than 70 years. During this period, both computing architecture and simulation techniques have evolved significantly. This cumulative thesis presents an overview of several recent methodologies that are introduced in and investigated with the molecular simulation tool ms2. The perspectives considered range from theoretical foundations in statistical mechanics to practical aspects of the implementation, as well as data analysis and parameter studies. The starting point is the Lennard-Jones potential which is well-known for its ability to model fluid equilibrium properties with good accuracy. While this is reconfirmed in a study on transcritical vapor-liquid equilibria of mixtures, the question remains whether interaction potentials with more adjustable parameters could yield better predictions. Therefore, a Tang-Toennies type potential with five parameters is introduced into ms2. After a complex parameter fitting procedure, however, it turns out that the limitations are due to the exclusive consideration of pairwise interactions, and cannot be remedied by additional parameters alone. Instead, it is necessary to take triplet interactions into account. Since this is associated with a significant increase in computational effort, a cutoff scheme and corresponding correction terms are developed. Over the course of four versions, the number of available statistical ensembles in ms2 was successively increased. These ensembles are part of a set of eight interrelated theories. In its latest release 5.0, ms2 has been completed in the sense that it now includes all eight of them. Each ensemble is associated with a thermodynamic potential that gives access to all equilibrium properties via its derivatives. The resulting properties are expected to coincide for all ensembles in the thermodynamic limit, which is confirmed in an extensive Monte Carlo simulation study. Another investigation shows that the thermodynamic potentials of different fluids exhibit similar topologies.