The solvation free energies for acetic acids in the anti/syn conformations in water solution have been computed by the novel QM/MM approach combined with the theory of energy representation (QM/MM-ER). To examine the accuracy of the methodology, we have compared the results with those given by experiments and other theoretical calculations. The solvation free energies computed by the QM/MM-ER approach are in reasonable agreement with corresponding experimental values. The free energy difference between the anti and syn conformers also agrees well with the results of AM1/TIP3P or RISM-SCF calculations. (c) 2005 Elsevier B.V. All rights reserved.