The ability of five different force fields (MM2, MM3, AMBER, OPLS, MMFF) to calculate relative conformational energies of seven molecules is compared to results at the MP2/aug-cc-pVTZ level. It is found that the quality of the results deteriorates as the polarity of the molecules increases for all force fields, strongly indicating that the use of fixed partial charges for representing the electrostatic interaction limits the accuracy that can be obtained.
