In many processes in the Earth’s crust and mantle fluids are involved. The formation of melts, the metasomatic alteration of rocks and more or less any material transport but also the rheological properties of minerals and rocks are influenced by the thermodynamic properties of fluids. Rocks, which are transferred to depth by tectonic processes, release fluids as the temperature increases. The compositions of such fluids can often be approximated in the system C-O-H and consist of mainly H2O and CO2. In many geological settings, however, the compositions of real fluids are much more complex and involve species like H2, CO, CH4, N2, NH3, HCl, H2S, SO2 and Ar. For the modeling of processes, which requires the thermodynamic treatment of such complex fluids at geologic conditions, equations of state (EOS) are needed. Whereas for simple systems like H2O-CO2-CH4 such EOS are available they do not exist for fluids of complex composition. The key for the development of EOS for complex fluids is the understanding of intermolecular potentials between various species. The potentials consider interactions by dispersion forces, dipole-, quadrupole- and other multipole interactions, induced multipoles forces as well as repulsion. One such equation is the EOS by Churakov & Gottschalk (2003). Under consideration of intermolecular forces and the use of the perturbation theory a general EOS for a large set of species is developed can be applied to complex fluid mixtures. However, the set of interactions used in this EOS is not complete and recalculated fluid miscibilities are only semi-quantitative. Ab-initio calculations, i.e. the calculation of electronic potentials on a molecular level considering the electrons of the outer shell, are able to predict precise energies for intermolecular interactions. However, for precise calculations this approach is limited to 3 to 4 molecules. On the basis of such calculations new EOS are developed.