A method for performing a simulation of a field having a subterranean formation is described. The method includes obtaining phase behavior data of subterranean fluids of the field, generating an equation of state (EOS) model of the fluids based on the phase behavior data, generating a Helmholtz free energy model that reproduces predictions of the EOS model over a pre-determined pressure and temperature range, and performing the simulation of the field using the Helmholtz free energy model. The method may further include reducing the EOS model to a reduced EOS model having a reduced number of components to represent the EOS model over a pre-determined pressure and temperature range, generating the Helmholtz free energy model based on the reduced EOS model, and obtaining and using phase behavior data of injection fluids used. A computer system data.