Calculating F-F spin-spin coupling constants across hydrogen bonds has represented a significant challenge to theory. In this study, ab initio calculations have been carried out to evaluate vibrational effects on the F-F spin-spin coupling constant ((2h)J(F-F)) for FHF-. The coupling-constant surface 2hJF-F was generated at EOM-CCSD/(qzp,qz2p), and two-dimensional wave functions for the symmetric and asymmetric stretching vibrations were obtained from the CCSD(T)/aug ' -cc-pVTZ potential surface. The effect of the FHF- bending mode was examined using one-dimensional calculations along the normal coordinate for the bending motion. Although (2h)J(F-F) is dominated by the Fermi-contact term in the region of the surface surrounding the equilibrium structure, the paramagnetic spin-orbit and spin dipole terms are important in determining the absolute value of (2h)J(F-F). In the ground vibrational state, the expectation value of the F-F distance increases, and the expectation value of (2h)J(F-F) decreases to 212.7 Hz, significantly less than the equilibrium value of 254.4 Hz: This decrease is due primarily to a decrease in the expectation value of the Fermi-contact term. The ground-state expectation value of the F-F coupling constant is consistent with an experimental estimate of 220 Hz, obtained by extrapolation of experimental values of (2h)J(F-F) for larger clusters of [F(HF)(n)](-). For FDF-, the expectation value of (2h)J(F-F) in the ground vibrational state is 223.1 Hz. Thermal vibrational averaging at 298 K over lower-energy excited vibrational states leas essentially no effect on (2h)J(F-F).