A simplified generalized Flory dimer (s-GFD) theory is developed to improve the performance of equation of state (EOS) for long chain molecules. The new EOS is formulated by a combination of the s-GFD theory and perturbed chain statistical associating fluid theory (PC-SAM. Three pure component parameters are required for nonassociating fluids: the number of segments per molecules (m), the diameter of segments (sigma), and the depth of intermolecular potential (epsilon/k). These parameters were obtained by correlating experimental liquid densities and vapor pressures for pure n-alkanes ranging from methane to eicosane. The pure component parameters of polyethylene (PE) were determined by extrapolating the pure component parameters of a series of n-alkanes. Comparisons to other EOSs are made up with the PC-SAFT and the statistical associating fluid theory for potentials of variable attractive range (SAFT-VR). The proposed equation provides excellent correlation results for vapor pressures and liquid densities, particularly for long chain molecules and for PE of a broad range of molecular weights. The proposed model allows phase equilibrium calculations to be conducted for binary and ternary mixtures of volatile components without requiring the use of binary interaction parameters. Correlations were carried out for equilibrium behaviors of PE solutions by using one adjustable parameter, and the calculated results agreed well with experimental data. (C) 2015 Elsevier B.V. All rights reserved.