A four dimensional (4D) time-dependent calculation to Obtain the.first vibrational states. of theliydrogen,bifluoride ion, FHF-, and its deUterated- counterpart, FDF-, has been performed using a spectral method in Cartesian coordinates. The corresponding potential energy surfaces have been computed at the CCSD(T)/aug-cc-pVTZ level of theory. The obtained values for the fundamental vibrational bands (nu) over tilde (1) = 589 cm(-1), (nu) over tilde (2) = 1305 cm(-1), and (nu) over tilde (3) = 1372 cm(-1) assigned to the symmetric stretch, bend, and asymmetric stretch modes, respectively (598, 943, and 972 cm(-1) for FDF-, respectively) are in good agreement with available experimental and theoretical vahies. Selected.overtones and mixed Modes are also calculated. Infrared spectra have been simulated Using the dipole approximation for two different polarization directions of the incident light.