The conformational distribution of flexible molecules may have a significant influence on its physical properties and phase behaviors. While atomistic molecular simulations naturally include conformational changes, they are often computationally expensive and require separate simulations for each state condition. Conversely, an equation of state (EOS) provides a rapid description for a variety of fluid properties over the whole phase space; however, because of the ignorance of molecular structure, it is difficult to obtain the interaction parameters from first-principles. Here we propose a multiscale approach for simultaneous predictions of fluid properties and molecular conformational distribution. The PR+COSMOSAC EOS is used to provide the fugacity of chemical species in a mixture. The electrostatic and dispersion interactions in the EOS are determined from quantum-mechanical solvation calculations and molecular dynamics simulations at a few state points. The conformational transitions are considered as chemical reactions with the equilibrium constant determined from ab initio G4 calculations. With all the EOS parameters obtained from molecular simulations at different scales, subsequent phase equilibrium predictions can be achieved within milliseconds. We validated this approach using 1,2-dichloroethane (DCE), whose dipole moment varies from 6.34 x 10(-30) Cm in the gauche-form to 0 in the trans-form. Our results show that this approach provides not only accurate predictions for the vapor-liquid and liquid-liquid equilibrium of DCE-containing fluids but also quantitative descriptions of conformational distribution of DCE in these systems. This novel method can be very useful for the prediction of thermodynamic properties of fluids with explicit inclusion of molecular conformations. (c) 2016 American Institute of Chemical Engineers AIChE J, 62: 4047-4054, 2016