The structure and dynamical behavior of liquid KF with dissolved KF have been investigated using the Car-Parrinello ab initio molecular dynamics scheme, Specifically, a system with stoichiometry KF.2HF was studied at temperatures of 400 and 1000 K. This system, which was started from a phase separated mixture, rapidly formed into solvated potassium ions and HnFn+1- polyfluoride anions with n = 1, 2, 3, and 4. The resulting polyfluoride anions were classified, and their structures and dynamical behavior were compared with the known structures and spectra of crystalline compounds KF.xHF and with theoretical predictions of isolated gas phase species, in the present study we reveal dramatic frequency shifts in the H atom vibrational modes with variation in the HF coordination number of the polyfluoride anion. In particular, the FH wagging motion red shifts while the FH stretch blue shifts as n increases. The present calculations suggest that FH stretching motion infrared modes previously assigned as combination bands should likely be reinterpreted as fundamental modes of the polyfluoride anions. (C) 1997 American Institute of Physics.