Using Monte Carlo free energy perturbation simulations, the entropy variation of the trans-gauche conformational equilibrium of 1,2-dichloroethane in water has been computed. Thermodynamic cycles have been used to evaluate the contribution of the van der Waals, electrostatic and polarization forces. Although the experimental free energy is reproduced, the simultaneous reproduction of the experimental free energy and entropy thermodynamic quantities cannot be obtained. The scaling of the solute atomic charge and polarizability values is not able to improve the results. To get proper entropy values for this conformational equilibrium, a new reparametrization involving both solute and solvent molecules is required. (c) 2005 Elsevier B.V. All rights reserved.