Ab initio calculations are used to calculate the atomic charges in the selectivity filter part of KcsA potassium channel according to the Merz-Kollman-Singh scheme. On the basis of a long molecular dynamics simulation, we show that these charges deviate significantly from the values usually implemented in the most common force fields. These consequent changes may considerably influence our views on the mechanistics of the potassium diffusion process and give strong evidence that the application of electronically polarizable force fields is necessary. (C) 2004 Elsevier B.V. All rights reserved.