Free energy is the criterion of stability and is essential for determining phase equilibrium properties, for example. However, calculation of free energies for complex systems, such as fluids by computer simulation, is extremely difficult. In this thesis, we show how the partition function of fluids can be calculated directly from simulations; this allows us to obtain the absolute Helmholtz free energy (F) via F- -k8TInQ. Our method radically simplifies the process of calculating absolute free energies of continuous systems. As the method has been developed in the past few months, we have not yet applied it to the study of phase equilibria. This task will be part of our future work. In the rest of the thesis, we have focused on the application of more established simulation techniques to the urgent problem of finding environmentally friendly refrigerant fluids. Methane and fluoromethanes are possible candidates. However, they are flammable. 1-1-1-2-tetrafluoroethane, on the other hand, has for a long time been used in domestic refrigeration and automobile air-conditioning systems. However, it will be banned in Europe from 2011, due to concerns about its global warming impact. Carbon dioxide has received much attention as a fluid that can be used in combination with other refrigerants to minimise flammability and toxicity, and has a very low global warming potential. Thus, it could be mixed with those refrigerants to form new environmentally friendly refrigerant mixtures. Unfortunately, little information on the thermophysical properties of these mixtures is available. We simulate the thermophysical properties of these important industrial refrigerants and their mixtures with carbon dioxide using both empirical and in-house firstprinciples potentials. Simulations also provide a microscopic-level understanding of the structure of liquids, which is not accessible via experiment. Our high-quality ab initio force fields have reproduced the thermophysical properties for carbon dioxide, methane, fluorinated methanes, and mixtures of carbon dioxide and methane and carbon dioxide and fluorinated methanes. Multi-body effects play a crucial role in determining the thermophysical properties of fluids and inclusion of a three-body effect substantially improves the prediction of the phase-coexistence properties. Our studies should be of relevance to a broad range of mixtures of fluoroalkanes and carbon dioxide. Our efforts in making the first-principle force fields for carbon dioxide and fluorinated methanes pave the way for larger fluorinated hydrocarbons to come in the future.